| 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 | - |
| 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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| inscrutable | (adjective) of an obscure nature | Synonyms: cryptic, cryptical, deep, mysterious, mystifying |
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| insensible | (adjective) incapable of physical sensation | - |
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| (adjective) barely able to be perceived | Synonyms: indiscernible, undetectable |
| (adjective) unresponsive to stimulation | Synonyms: out of it, senseless |
| (adjective) unaware of or indifferent to | Synonyms: unaffected |
| inseparable | (adjective) not capable of being separated | - |
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| insoluble | (adjective) (of a substance) incapable of being dissolved | Synonyms: indissoluble |
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| (adjective) admitting of no solution or explanation | - |
| (adjective) without hope of solution | - |
| insolvable | (adjective) not easily solved | Synonyms: unresolvable, unsoluble, unsolvable |
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| insufferable | (adjective) unbearably arrogant or conceited | - |
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| (adjective) too extreme to bear | Synonyms: unsufferable |
| insuperable | (adjective) incapable of being surmounted or excelled | Synonyms: unconquerable |
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| (adjective) impossible to surmount | Synonyms: insurmountable |
| insupportable | (adjective) incapable of being justified or explained | Synonyms: indefensible, unjustifiable, unwarrantable, unwarranted |
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| insurable | (adjective) capable of being insured or eligible to be insured | - |
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| insurmountable | (adjective) not capable of being surmounted or overcome | Synonyms: unsurmountable |
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| (adjective) impossible to surmount | Synonyms: insuperable |
| insusceptible | (adjective) not susceptible to | Synonyms: unsusceptible |
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| intangible | (adjective) incapable of being perceived by the senses especially the sense of touch | Synonyms: impalpable |
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| (adjective) (of especially business assets) not having physical substance or intrinsic productive value | - |
| (adjective) lacking substance or reality; incapable of being touched or seen | Synonyms: nonphysical |
| (adjective) hard to pin down or identify | - |
| intelligible | (adjective) well articulated or enunciated, and loud enough to be heard distinctly | - |
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| (adjective) capable of being apprehended or understood | Synonyms: apprehensible, graspable, perceivable, understandable |
| interchangeable | (adjective) capable of replacing or changing places with something else; permitting mutual substitution without loss of function or suitability | Synonyms: exchangeable, similar, standardised, standardized |
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| (adjective) (mathematics, logic) such that the arguments or roles can be interchanged | - |
| interminable | (adjective) tiresomely long; seemingly without end | Synonyms: endless, eternal |
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| interoperable | (adjective) able to exchange and use information | - |
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| interpretable | (adjective) capable of being understood | Synonyms: explainable |
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| intolerable | (adjective) incapable of being tolerated or endured | Synonyms: unbearable, unendurable |
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| intractable | (adjective) not tractable; difficult to manage or mold | - |
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| introuvable | (adjective) impossible to find | - |
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| invaluable | (adjective) having incalculable monetary, intellectual, or spiritual worth | Synonyms: priceless |
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| invariable | (adjective) not liable to or capable of change | - |
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| invertible | (adjective) having an additive or multiplicative inverse | - |
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| invincible | (adjective) incapable of being overcome or subdued | Synonyms: unbeatable, unvanquishable |
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| inviolable | (adjective) incapable of being transgressed or dishonored | - |
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| (adjective) not capable of being violated or infringed | Synonyms: absolute, infrangible |
| (adjective) immune to attack; incapable of being tampered with | Synonyms: impregnable, secure, strong, unassailable, unattackable |
| (adjective) must be kept sacred | Synonyms: inviolate, sacrosanct |
| invisible | (adjective) impossible or nearly impossible to see; imperceptible by the eye | Synonyms: unseeable |
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| (adjective) not prominent or readily noticeable | Synonyms: inconspicuous |
| invulnerable | (adjective) immune to attack; impregnable | - |
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| irascible | (adjective) characterized by anger | Synonyms: choleric |
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| (adjective) quickly aroused to anger | Synonyms: choleric, hot-tempered, hotheaded, quick-tempered, short-tempered |
| irreclaimable | (adjective) insusceptible of reform | Synonyms: irredeemable, unredeemable, unreformable |
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| irreconcilable | (adjective) impossible to reconcile | Synonyms: unreconcilable |
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| irrecoverable | (adjective) incapable of being recovered or regained | Synonyms: unrecoverable |
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| irredeemable | (adjective) (of paper money) not convertible into coin at the pleasure of the holder | - |
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| (adjective) insusceptible of reform | Synonyms: irreclaimable, unredeemable, unreformable |
| irreducible | (adjective) incapable of being made smaller or simpler | - |
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| irrefutable | (adjective) impossible to deny or disprove | Synonyms: incontrovertible, positive |
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| irremediable | (adjective) impossible to remedy or correct or redress | - |
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| irremovable | (adjective) incapable of being removed or away or dismiss | - |
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| irreparable | (adjective) impossible to repair, rectify, or amend | - |
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| irreplaceable | (adjective) impossible to replace | Synonyms: unreplaceable |
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| irrepressible | (adjective) impossible to repress or control | Synonyms: uncontrollable |
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| irreproachable | (adjective) free of guilt; not subject to blame | Synonyms: blameless, inculpable, unimpeachable |
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| irreproducible | (adjective) impossible to reproduce or duplicate | Synonyms: unreproducible |
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| irresistible | (adjective) impossible to resist; overpowering | Synonyms: resistless |
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| (adjective) overpoweringly attractive | - |
| irresponsible | (adjective) showing lack of care for consequences | - |
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