imprisonment (n.) Look up imprisonment at Dictionary.com
late 14c., from Anglo-French emprisonement, Old French emprisonement (13c.), from emprisoner (see imprison).
improbability (n.) Look up improbability at Dictionary.com
1590s, "fact or quality of being improbably;" see improbable + -ity. Meaning "an instance of something improbable" is from 1610s.
improbable (adj.) Look up improbable at Dictionary.com
1590s, from assimilated form of in- (1) "not, opposite of" + probable, or else from Latin improbabilis. Related: Improbably.
imprompt (adj.) Look up imprompt at Dictionary.com
(obsolete) 1759, from Latin impromptus "unready, hesitating," from assimilated form of in- "not, opposite of" (see in- (1)) + promptus "ready" (see prompt).
impromptu Look up impromptu at Dictionary.com
1660s (adv.), 1764 (adj.), from French impromptu (1650s), from Latin in promptu "in readiness," from assimilated form of in- "into, in, on, upon" (see in- (2)) + promptu, ablative of promptus "readiness," from past participle of promere "to bring out," from pro- "before, forward, for" + emere "to obtain" (see exempt).
improper (adj.) Look up improper at Dictionary.com
mid-15c., "not true," from French impropre (14c.), from Latin improprius, from assimilated form of in- "not, opposite of" (see in- (1)) + proprius (see proper). Meaning "not suited, unfit" is from 1560s; that of "not in accordance with good manners, modesty, decency" is from 1739. Related: Improperly (late 14c.).
impropriety (n.) Look up impropriety at Dictionary.com
1610s, "quality or fact of being improper," from French impropriété (16c.), from Latin improprietas, from improprius (see improper). As "improper thing," 1670s.
improv (n.) Look up improv at Dictionary.com
1970 as colloquial shortening for improvisation. The New York City comedy club, founded in 1963, was, in full, The Improvisation.
improve (v.) Look up improve at Dictionary.com
late 15c., "to use to one's profit, to increase (income)," from Anglo-French emprouwer "to turn to profit" (late 13c.), from Old French en-, causative prefix, + prou "profit," from Latin prode "advantageous" (see proud). Spelling with -v- was rare before 17c. Meaning "to raise to a better quality or condition" first recorded 1610s. Phrase improve the occasion retains the etymological sense. Meaning "to turn land to profit" (by clearing it, erecting buildings, etc.) was in Anglo-French (13c.) and was retained in the American colonies.
improvement (n.) Look up improvement at Dictionary.com
mid-15c., enprowment "management of something for profit," from Anglo-French emprowement, from emprouwer "turn to profit" (see improve). Meaning "betterment; amelioration" is from 1640s. Meaning "buildings, etc. on a piece of property" is from 1773. Related: Improvements.
improvidence (n.) Look up improvidence at Dictionary.com
"lack of foresight, rashness," mid-15c., from Latin improvidentia, from assimilated form of in- "not" (see in- (1)) + providentia (see providence).
improvident (adj.) Look up improvident at Dictionary.com
1510s, from im- "not" + provident. It retains a stronger connection with the "provide" aspect of Latin providere. Related: Improvidently.
improvisation (n.) Look up improvisation at Dictionary.com
mid-15c., "unforeseen happening;" 1786 as "act of improvising musically," from French improvisation, from improviser "compose or say extemporaneously," from Italian improvvisare, from improvviso "unforeseen, unprepared," from Latin improvisus "not foreseen, unforeseen, unexpected," from assimilated form of in- "not, opposite of" (see in- (1)) + provisus "foreseen," also "provided," past participle of providere "foresee, provide" (see provide).
improvisational (adj.) Look up improvisational at Dictionary.com
1879; see improvisation + -al (1).
improvise (v.) Look up improvise at Dictionary.com
1826, back-formation from improvisation, or else from French improviser (17c.), from Italian improvisare "to sing or speak extempore," from improviso, from Latin improvisus "unforeseen, unexpected" (see improvisation). Or possibly a back-formation from improvisation. Related: Improvised; improvising.
improvision (n.) Look up improvision at Dictionary.com
"want of forethought," 1640s, from assimilated form of in- (1) "not, opposite of" + provision.
imprudence (n.) Look up imprudence at Dictionary.com
early 15c., "quality of rashness or heedlessness; imprudent act," from Latin imprudentia "lack of foresight, inconsiderateness, ignorance, inadvertence," noun of quality from imprudens (see imprudent).
imprudent (adj.) Look up imprudent at Dictionary.com
late 14c., from Latin imprudentem (nominative imprudens) "not foreseeing, unaware, inconsiderate, heedless," from assimilated form of in- "not, opposite of" (see in- (1)) + prudens, contraction of providens, present participle of providere "to provide," literally "to see before (one)" (see provide). Related: Imprudently.
impudence (n.) Look up impudence at Dictionary.com
late 14c., from Latin impudentia "shamelessness," noun of quality from impudens; see impudent.
impudent (adj.) Look up impudent at Dictionary.com
late 14c., from Latin impudentem (nominative impudens) "without shame, shameless," from assimilated form of in- "not, opposite of" (see in- (1)) + pudens "ashamed, modest," present participle of pudere "to cause shame" (see pudendum). Related: Impudently.
impugn (v.) Look up impugn at Dictionary.com
"attack by argument," late 14c., from Old French impugner, from Latin impugnare "to assault, to attack," from assimilated form of in- "into, in, on, upon" (see in- (2)) + pugnare "to fight" (see pugnacious). Related: Impugned; impugning.
impulse (n.) Look up impulse at Dictionary.com
early 15c., "an act of impelling, a thrust, push," from Latin impulsus "a push against, pressure, shock," also "incitement, instigation, impulse," past participle of impellere (see impel). Meaning "stimulus in the mind arising from some state or feeling" first recorded 1640s.
impulsion (n.) Look up impulsion at Dictionary.com
early 15c., "driving, pushing, thrusting," from Old French impulsion (early 14c.), from Latin impulsionem (nominative impulsio) "external pressure," figuratively "incitement, instigation," noun of action from past participle stem of impellere (see impel).
impulsive (adj.) Look up impulsive at Dictionary.com
early 15c., originally in reference to medicine that reduces swelling or humors, from Middle French impulsif or directly from Medieval Latin impulsivus, from Latin impuls-, past participle stem of impellere (see impel). Of persons, "rash, characterized by impulses," from 1847.
impulsively (adv.) Look up impulsively at Dictionary.com
1768; see impulsive + -ly (2).
impulsiveness (n.) Look up impulsiveness at Dictionary.com
1650s; see impulsive + -ness.
impulsivity (n.) Look up impulsivity at Dictionary.com
1891; see impulsive + -ity.
impune (adj.) Look up impune at Dictionary.com
"unpunished" (obsolete), 1610s, from Latin impunis "unpunished," from assimilated form of in- "not, opposite of" (see in- (1)) + poena (see penal).
impunity (n.) Look up impunity at Dictionary.com
1530s, from Middle French impunité (14c.) and directly from Latin impunitatem (nominative impunitas) "freedom from punishment, omission of punishment," also "rashness, inconsideration," from impunis "unpunished, without punishment," from assimilated form of in- "not, opposite of" (see in- (1)) + poena "punishment" (see penal).
impure (adj.) Look up impure at Dictionary.com
mid-15c., from Middle French impur (13c.), from Latin impurus "unclean, filthy, foul," from assimilated form of in- "not, opposite of" (see in- (1)) + purus "pure" (see pure). As a noun from 1784. Related: Impurely.
impurity (n.) Look up impurity at Dictionary.com
mid-15c., "thing which makes or is impure;" c.1500, "fact or quality of being impure," from Middle French impurité, from impur (see impure). Related: Impurities.
imputable (adj.) Look up imputable at Dictionary.com
1620s, from Medieval Latin imputabilis, from Latin imputare (see impute).
imputation (n.) Look up imputation at Dictionary.com
1540s, noun of action from impute (v.) on model of Middle French imputation, or else from Late Latin imputationem (nominative imputatio), noun of action from imputare.
impute (v.) Look up impute at Dictionary.com
early 15c., from Old French imputer (14c.) and directly from Latin imputare "to reckon, make account of, charge, ascribe," from assimilated form of in- "in, into" (see in- (2)) + putare "reckon, clear up, trim, prune, settle" (see pave). Related: Imputed; imputing.
in Look up in at Dictionary.com
Old English in (prep.) "in, into, upon, on, at, among; about, during;" inne (adv.) "within, inside," from Proto-Germanic *in (cognates: Old Frisian, Dutch, German, Gothic in, Old Norse i), from PIE *en "in" (cognates: Greek en, Latin in "in, into," Old Irish in, Welsh yn-, Old Church Slavonic on-). As an adjective from 1590s.

The forms merged in Middle English. Modern sense distinction between in and on is from later Middle English. Sense of "holding power" (the in party) first recorded c.1600; that of "exclusive" (the in-crowd, an in-joke) is from 1907 (in-group); that of "stylish, fashionable" (the in thing) is from 1960. The noun sense of "influence, access" (have an in with) first recorded 1929 in American English. In-and-out "copulation" is attested from 1610s.
in absentia Look up in absentia at Dictionary.com
Latin, literally "in (his/her/their) absence" (see absence).
in extremis Look up in extremis at Dictionary.com
"at the point of death," Latin, literally "in the farthest reaches."
in like Flynn Look up in like Flynn at Dictionary.com
1940s slang, said to have originated in the U.S. military, perhaps from alleged sexual exploits of Hollywood actor Errol Flynn.
in loco parentis Look up in loco parentis at Dictionary.com
Latin, literally "in the place of a parent" (see parent).
in medias res Look up in medias res at Dictionary.com
Latin, literally "in the midst of things" (see medium).
in memoriam Look up in memoriam at Dictionary.com
Latin, literally "in memory of" (see memory).
in situ Look up in situ at Dictionary.com
Latin, literally "in its (original) place or position" (see situate (v.)).
in toto Look up in toto at Dictionary.com
Latin, "as a whole, completely" (see total).
in utero Look up in utero at Dictionary.com
Latin, literally "in the uterus" (see uterus).
in vitro Look up in vitro at Dictionary.com
Latin; "in a test tube, culture dish, etc.;" literally "in glass" (see vitreous).
in vivo Look up in vivo at Dictionary.com
Latin; "within a living organism" (see viva).
in't Look up in't at Dictionary.com
archaic; 17c. as short for in it.
in't Look up in't at Dictionary.com
archaic or poetic contraction of in it.
in- (1) Look up in- at Dictionary.com
prefix meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant), from Latin in- "not," cognate with Greek an-, Old English un-, from PIE *ne "not" (see un- (1)).
in- (2) Look up in- at Dictionary.com
element meaning "into, in, on, upon" (also im-, il-, ir- by assimilation of -n- with following consonant), from Latin in- "in" (see in). In Old French this often became en-, which usually was respelled in English to conform with Latin, but not always, which accounts for pairs like enquire/inquire. There was a native form, which in West Saxon usually appeared as on- (as in Old English onliehtan "to enlighten"), and some verbs survived into Middle English (such as inwrite "to inscribe"), but all now seem to be extinct. Not related to in- (1) "not," which also was a common prefix in Latin: to the Romans impressus could mean "pressed" or "unpressed."