| impracticable | (adjective) not capable of being carried out or put into practice | Synonyms: infeasible, unfeasible, unworkable |
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| impractical | (adjective) not practical; not workable or not given to practical matters | - |
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| (adjective) not practical or realizable; speculative | Synonyms: airy, Laputan, visionary, windy |
| imprecise | (adjective) not precise | - |
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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 |
| impressed | (adjective) deeply or markedly affected or influenced | - |
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| 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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| impressionist | (adjective) relating to or characteristic of Impressionism | Synonyms: impressionistic |
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| impressionistic | (adjective) relating to or characteristic of Impressionism | Synonyms: impressionist |
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| (adjective) of or relating to or based on an impression rather than on facts or reasoning | - |
| impressive | (adjective) making a strong or vivid impression | - |
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| (adjective) producing a strong effect | Synonyms: telling |
| imprisoned | (adjective) being in captivity | Synonyms: captive, confined, jailed |
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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 |
| impromptu | (adjective) with little or no preparation or forethought | Synonyms: ad-lib, extemporaneous, extemporary, extempore, off-the-cuff, offhand, offhanded, unrehearsed |
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| improper | (adjective) not suitable or right or appropriate | - |
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| (adjective) not appropriate for a purpose or occasion | Synonyms: wrong |
| (adjective) not conforming to legality, moral law, or social convention | Synonyms: unconventional, unlawful |
| improvable | (adjective) susceptible of improvement | - |
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| improved | (adjective) made more desirable or valuable or profitable; especially made ready for use or marketing | - |
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| (adjective) become or made better in quality | - |
| (adjective) (of land) made ready for development or agriculture by clearing of trees and brush | - |
| improvident | (adjective) not provident; not providing for the future | - |
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| (adjective) not given careful consideration | Synonyms: ill-considered, ill-judged, shortsighted |
| improving | (adjective) getting higher or more vigorous | Synonyms: up |
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| improvised | (adjective) done or made using whatever is available | Synonyms: jury-rigged, makeshift |
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| imprudent | (adjective) not prudent or wise | - |
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| (adjective) lacking wise self-restraint | - |