imprisonment (n.)
late 14c., from Anglo-French emprisonement, Old French emprisonement (13c.), from emprisoner (see imprison).
improbability (n.)
1590s, "fact or quality of being improbably;" see improbable + -ity. Meaning "an instance of something improbable" is from 1610s.
improbable (adj.)
1590s, from assimilated form of in- (1) "not, opposite of" + probable, or else from Latin improbabilis. Related: Improbably.
imprompt (adj.)
(obsolete) 1759, from Latin impromptus "unready, hesitating," from assimilated form of in- "not, opposite of" (see in- (1)) + promptus "ready" (see prompt).
impromptu
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.)
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.)
1610s, "quality or fact of being improper," from French impropriété (16c.), from Latin improprietas, from improprius (see improper). As "improper thing," 1670s.
improv (n.)
1970 as colloquial shortening for improvisation. The New York City comedy club, founded in 1963, was, in full, The Improvisation.
improve (v.)
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.)
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.)
"lack of foresight, rashness," mid-15c., from Latin improvidentia, from assimilated form of in- "not" (see in- (1)) + providentia (see providence).
improvident (adj.)
1510s, from im- "not" + provident. It retains a stronger connection with the "provide" aspect of Latin providere. Related: Improvidently.
improvisation (n.)
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.)
1879; see improvisation + -al (1).
improvise (v.)
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.)
"want of forethought," 1640s, from assimilated form of in- (1) "not, opposite of" + provision.
imprudence (n.)
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.)
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.)
late 14c., from Latin impudentia "shamelessness," noun of quality from impudens; see impudent.
impudent (adj.)
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.)
"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.)
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.)
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.)
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.)
1768; see impulsive + -ly (2).
impulsiveness (n.)
1650s; see impulsive + -ness.
impulsivity (n.)
1891; see impulsive + -ity.
impune (adj.)
"unpunished" (obsolete), 1610s, from Latin impunis "unpunished," from assimilated form of in- "not, opposite of" (see in- (1)) + poena (see penal).
impunity (n.)
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.)
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.)
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.)
1620s, from Medieval Latin imputabilis, from Latin imputare (see impute).
imputation (n.)
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.)
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
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
Latin, literally "in (his/her/their) absence" (see absence).
in extremis
"at the point of death," Latin, literally "in the farthest reaches."
in like Flynn
1940s slang, said to have originated in the U.S. military, perhaps from alleged sexual exploits of Hollywood actor Errol Flynn.
in loco parentis
Latin, literally "in the place of a parent" (see parent).
in medias res
Latin, literally "in the midst of things" (see medium).
in memoriam
Latin, literally "in memory of" (see memory).
in situ
Latin, literally "in its (original) place or position" (see situate (v.)).
in toto
Latin, "as a whole, completely" (see total).
in utero
Latin, literally "in the uterus" (see uterus).
in vitro
Latin; "in a test tube, culture dish, etc.;" literally "in glass" (see vitreous).
in vivo
Latin; "within a living organism" (see viva).
in't
archaic; 17c. as short for in it.
in't
archaic or poetic contraction of in it.
in- (1)
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)
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."