| impossible | (adjective) not capable of occurring or being accomplished or dealt with | - |
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| (adjective) (used of persons or their behavior) not acceptable or reasonable | Synonyms: unacceptable |
| (adjective) totally unlikely | Synonyms: inconceivable, out of the question, unimaginable |
| (noun) something that cannot be done | - |
| impracticable | (adjective) not capable of being carried out or put into practice | Synonyms: infeasible, unfeasible, unworkable |
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| impregnable | (adjective) capable of conceiving | Synonyms: conceptive |
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| (adjective) immune to attack; incapable of being tampered with | Synonyms: inviolable, secure, strong, unassailable, unattackable |
| (adjective) incapable of being overcome, challenged or refuted | Synonyms: inexpugnable |
| impressible | (adjective) easily impressed or influenced | Synonyms: impressionable, waxy |
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| impressionable | (adjective) easily impressed or influenced | Synonyms: impressible, waxy |
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| improbable | (adjective) not likely to be true or to occur or to have occurred | Synonyms: unlikely |
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| (adjective) having a probability too low to inspire belief | Synonyms: unbelievable, unconvincing, unlikely |
| (adjective) too improbable to admit of belief | Synonyms: marvellous, marvelous, tall |
| improvable | (adjective) susceptible of improvement | - |
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| impugnable | (adjective) subject to being discredited | - |
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| imputable | (adjective) capable of being assigned or credited to | Synonyms: ascribable, due, referable |
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| imputrescible | (adjective) not subject to decay | - |
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| inaccessible | (adjective) capable of being reached only with great difficulty or not at all | Synonyms: unaccessible |
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| (adjective) not capable of being obtained | Synonyms: unobtainable, unprocurable, untouchable |
| inadmissible | (adjective) not deserving to be admitted | - |
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| inadvisable | (adjective) not prudent or wise; not recommended | Synonyms: unadvisable |
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| (adjective) not advisable | - |
| inalienable | (adjective) incapable of being repudiated or transferred to another | Synonyms: unalienable |
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| (adjective) not subject to forfeiture | Synonyms: unforfeitable |
| inalterable | (adjective) not capable of being changed or altered | Synonyms: unalterable |
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| inapplicable | (adjective) not capable of being applied | Synonyms: unsuitable |
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| inappreciable | (adjective) too small to make a significant difference | - |
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| inarguable | (adjective) against which no argument can be made | Synonyms: unarguable |
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| inaudible | (adjective) impossible to hear; imperceptible by the ear | Synonyms: unhearable |
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| incalculable | (adjective) not able to be computed or enumerated | - |
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| (adjective) too much to be measured | Synonyms: untold |
| incapable | (adjective) (followed by `of') lacking capacity or ability | - |
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| (adjective) (followed by `of') not having the temperament or inclination for | - |
| (adjective) not meeting requirements | Synonyms: incompetent, unequal to |
| (adjective) not being susceptible to or admitting of something (usually followed by `of') | - |
| incognizable | (adjective) incapable of being perceived or known | Synonyms: incognoscible |
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| incognoscible | (adjective) incapable of being perceived or known | Synonyms: incognizable |
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| incombustible | (adjective) not capable of igniting and burning | Synonyms: noncombustible |
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| incommensurable | (adjective) not having a common factor | - |
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| (adjective) impossible to measure or compare in value or size or excellence | - |
| incommutable | (adjective) not subject to alteration or change | - |
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| (adjective) not interchangeable or able to substitute one for another | - |
| incomparable | (adjective) such that comparison is impossible; unsuitable for comparison or lacking features that can be compared | Synonyms: uncomparable |
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| incompatible | (adjective) used especially of drugs or muscles that counteract or neutralize each other's effect | Synonyms: antagonistic |
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| (adjective) not compatible | - |
| (adjective) incapable of being used with or connected to other devices or components without modification | - |
| (adjective) not suitable to your tastes or needs | Synonyms: uncongenial |
| (adjective) of words so related that one contrasts with the other | Synonyms: contrastive |
| (adjective) used especially of solids or solutions; incapable of blending into a stable homogeneous mixture | - |
| (adjective) not in keeping with what is correct or proper | Synonyms: inappropriate, out of keeping, unfitting |
| (adjective) not compatible with other facts | Synonyms: discrepant |
| (adjective) not easy to combine harmoniously | Synonyms: ill-sorted, mismated, unsuited |
| incomprehensible | (adjective) difficult to understand | Synonyms: uncomprehensible |
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| (adjective) incapable of being explained or accounted for | Synonyms: inexplicable |
| incompressible | (adjective) incapable of being compressed; resisting compression | - |
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| incomputable | (adjective) beyond calculation or measure | Synonyms: immeasurable, inestimable |
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| inconceivable | (adjective) totally unlikely | Synonyms: impossible, out of the question, unimaginable |
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| inconsiderable | (adjective) too small or unimportant to merit attention | - |
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| inconsolable | (adjective) sad beyond comforting; incapable of being consoled | Synonyms: disconsolate, unconsolable |
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| incontestable | (adjective) incapable of being contested or disputed | Synonyms: incontestible |
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| (adjective) not open to question; obviously true | Synonyms: indisputable, undisputable |
| incontestible | (adjective) incapable of being contested or disputed | Synonyms: incontestable |
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| incontrovertible | (adjective) necessarily or demonstrably true | Synonyms: demonstrable |
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| (adjective) impossible to deny or disprove | Synonyms: irrefutable, positive |
| inconvertible | (adjective) used especially of currencies; incapable of being exchanged for or replaced by another currency of equal value | Synonyms: unconvertible, unexchangeable |
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| (adjective) not capable of being changed into something else | Synonyms: untransmutable |
| incorrigible | (adjective) impervious to correction by punishment | - |
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| (adjective) difficult or impossible to manage or control | - |
| incorruptible | (adjective) incapable of being morally corrupted | - |
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| incredible | (adjective) beyond belief or understanding | Synonyms: unbelievable |
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| inculpable | (adjective) free of guilt; not subject to blame | Synonyms: blameless, irreproachable, unimpeachable |
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| incurable | (adjective) incapable of being cured | - |
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| (adjective) unalterable in disposition or habits | - |
| (noun) a person whose disease is incurable | - |
| indecipherable | (adjective) not easily deciphered | Synonyms: unclear, undecipherable, unreadable |
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| (adjective) impossible to determine the meaning of | - |
| indefatigable | (adjective) showing sustained enthusiastic action with unflagging vitality | Synonyms: tireless, unflagging, unwearying |
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| indefeasible | (adjective) not liable to being annulled or voided or undone | - |
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| indefensible | (adjective) incapable of being justified or explained | Synonyms: insupportable, unjustifiable, unwarrantable, unwarranted |
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| (adjective) (of theories etc) incapable of being defended or justified | Synonyms: untenable |
| (adjective) not able to be protected against attack | - |
| indefinable | (adjective) defying expression or description | Synonyms: indescribable, ineffable, unspeakable, untellable, unutterable |
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| (adjective) not capable of being precisely or readily described; not easily put into words | Synonyms: undefinable |
| indelible | (adjective) cannot be removed or erased | Synonyms: unerasable |
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| indescribable | (adjective) defying expression or description | Synonyms: indefinable, ineffable, unspeakable, untellable, unutterable |
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| indestructible | (adjective) not easily destroyed | - |
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| (adjective) very long lasting | Synonyms: durable, perdurable, undestroyable |
| indeterminable | (adjective) not capable of being definitely decided or ascertained | Synonyms: undeterminable |
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| (adjective) incapable of being definitely ascertained or calculated | - |
| indictable | (adjective) liable to be accused, or cause for such liability | Synonyms: chargeable |
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| indigestible | (adjective) digested with difficulty | - |
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| indiscernible | (adjective) difficult or impossible to perceive or discern | - |
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| (adjective) barely able to be perceived | Synonyms: insensible, undetectable |
| indispensable | (adjective) not to be dispensed with; essential | - |
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| (adjective) absolutely necessary; vitally necessary | Synonyms: essential |
| (adjective) unavoidable | - |
| indisputable | (adjective) impossible to doubt or dispute | Synonyms: sure |
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| (adjective) not open to question; obviously true | Synonyms: incontestable, undisputable |
| indissoluble | (adjective) (of a substance) incapable of being dissolved | Synonyms: insoluble |
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| (adjective) used of decisions and contracts | - |
| indistinguishable | (adjective) not capable of being distinguished or differentiated | Synonyms: undistinguishable |
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| (adjective) exactly alike; incapable of being perceived as different | Synonyms: identical |
| indivisible | (adjective) impossible of undergoing division | - |
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| indomitable | (adjective) impossible to subdue | Synonyms: never-say-die, unsubduable |
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| indubitable | (adjective) too obvious to be doubted | Synonyms: beyond doubt |
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| inedible | (adjective) not suitable for food | Synonyms: uneatable |
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| ineffable | (adjective) defying expression or description | Synonyms: indefinable, indescribable, unspeakable, untellable, unutterable |
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| (adjective) too sacred to be uttered | Synonyms: unnameable, unspeakable, unutterable |
| ineligible | (adjective) not eligible | - |
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| (adjective) prohibited by official rules | - |
| ineluctable | (adjective) impossible to avoid or evade | Synonyms: inescapable, unavoidable |
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| inequitable | (adjective) not equitable or fair | Synonyms: unjust |
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| ineradicable | (adjective) not able to be destroyed or rooted out | - |
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| inerrable | (adjective) not liable to error | Synonyms: inerrant, unerring |
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| inescapable | (adjective) impossible to avoid or evade | Synonyms: ineluctable, unavoidable |
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| inestimable | (adjective) beyond calculation or measure | Synonyms: immeasurable, incomputable |
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| inevitable | (adjective) incapable of being avoided or prevented | - |
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| (adjective) invariably occurring or appearing | - |
| (noun) an unavoidable event | - |
| inexcusable | (adjective) without excuse or justification | - |
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| (adjective) not excusable | Synonyms: unforgivable |
| inexhaustible | (adjective) incapable of being entirely consumed or used up | - |
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| (adjective) that cannot be entirely consumed or used up | Synonyms: unlimited |
| inexorable | (adjective) not to be placated or appeased or moved by entreaty | Synonyms: grim, relentless, stern, unappeasable, unforgiving, unrelenting |
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| (adjective) impervious to pleas, persuasion, requests, reason | Synonyms: adamant, adamantine, intransigent |
| inexpiable | (adjective) incapable of being atoned for | - |
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| inexplicable | (adjective) incapable of being explained or accounted for | Synonyms: incomprehensible |
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| inexpressible | (adjective) defying expression | Synonyms: unexpressible |
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| inexpugnable | (adjective) incapable of being overcome, challenged or refuted | Synonyms: impregnable |
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| inexpungeable | (adjective) not capable of being expunged | Synonyms: inexpungible |
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| inexpungible | (adjective) not capable of being expunged | Synonyms: inexpungeable |
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| inextensible | (adjective) not extensile | Synonyms: nonextensile, nonprotractile |
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| inexterminable | (adjective) incapable of extermination or extirpation | Synonyms: inextirpable |
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| inextinguishable | (adjective) difficult or impossible to extinguish | - |
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| inextirpable | (adjective) incapable of extermination or extirpation | Synonyms: inexterminable |
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| inextricable | (adjective) not permitting extrication; incapable of being disentangled or untied | - |
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| infallible | (adjective) incapable of failure or error | - |
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| infeasible | (adjective) not capable of being carried out or put into practice | Synonyms: impracticable, unfeasible, unworkable |
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| inflammable | (adjective) easily ignited | Synonyms: flammable |
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| inflatable | (adjective) designed to be filled with air or gas | - |
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| inflexible | (adjective) resistant to being bent | - |
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| (adjective) incapable of change | - |
| (adjective) not making concessions | Synonyms: sturdy, uncompromising |
| (adjective) incapable of adapting or changing to meet circumstances | Synonyms: rigid, unbending |
| infrangible | (adjective) not capable of being violated or infringed | Synonyms: absolute, inviolable |
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| (adjective) difficult or impossible to break or separate into parts | - |
| inhabitable | (adjective) fit for habitation | Synonyms: habitable |
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| inheritable | (adjective) capable of being inherited | Synonyms: heritable |
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| inhospitable | (adjective) unfavorable to life or growth | - |
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| (adjective) not hospitable | - |
| inimitable | (adjective) defying imitation; matchless | - |
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| injectable | (adjective) (used of drugs) capable of being injected | - |
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| innumerable | (adjective) too numerous to be counted | Synonyms: countless, infinite, innumerous, multitudinous, myriad, numberless, uncounted, unnumberable, unnumbered, unnumerable |
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| inoperable | (adjective) not suitable for surgery | - |
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| (adjective) not able to perform its normal function | - |
| insatiable | (adjective) impossible to satisfy | Synonyms: insatiate, unsatiable |
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