| insubordinate | (adjective) not submissive to authority | - |
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| (adjective) disposed to or engaged in defiance of established authority | Synonyms: resistant, resistive |
| insubstantial | (adjective) lacking material form or substance; unreal | Synonyms: unreal, unsubstantial |
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| (adjective) lacking in nutritive value | - |
| (adjective) lacking solidity or strength | Synonyms: flimsy |
| insufferable | (adjective) unbearably arrogant or conceited | - |
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| (adjective) too extreme to bear | Synonyms: unsufferable |
| insufficient | (adjective) of a quantity not able to fulfill a need or requirement | Synonyms: deficient |
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| insular | (adjective) relating to or characteristic of or situated on an island | - |
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| (adjective) suggestive of the isolated life of an island | - |
| (adjective) narrowly restricted in outlook or scope | Synonyms: parochial |
| insulting | (adjective) expressing extreme contempt | Synonyms: contemptuous, disdainful, scornful |
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| 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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| insured | (adjective) covered by insurance | - |
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| insurgent | (adjective) in opposition to a civil authority or government | Synonyms: seditious, subversive |
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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 |
| insurrectional | (adjective) of or relating to or given to insurrection | Synonyms: insurrectionary |
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| insurrectionary | (adjective) of or relating to or given to insurrection | Synonyms: insurrectional |
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| insusceptible | (adjective) not susceptible to | Synonyms: unsusceptible |
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