Online Etymology Dictionary

imperishable (adj.): "not subject to destruction or decay," 1640s, from assimilated form of in- "not, opposite of" (see in- (1)) + perishable. Related: Imperishably
impermanence (n.): 1796, from impermanent + -ence. Impermanency is from 1640s.
impermanent (adj.): 1650s, from assimilated form of in- "not, opposite of" (see in- (1)) + permanent.
impermissible (adj.): 1814, from assimilated form of in- "not, opposite of" (see in- (1)) + permissible.
impiety (n.): mid-14c., from Old French impieté "impiety, wickedness" (12c.) or directly from Latin impietatem (nominative impietas) "irreverence, ungodliness; disloyalty, treason," noun of quality from impius "irreverent" (see impious).
impious (adj.): 1590s, "irreligious, lacking reverence for God," from Latin impius "without reverence, irreverent, wicked; undutiful, unpatriotic," from assimilated form of in- "not, opposite of" (see in- (1)) + pius (see pious). Related: Impiously; impiousness.
impish (adj.): 1650s, from imp + -ish. Related: Impishly; impishness.
impolitic (adj.): "not according to good policy," c. 1600, from assimilated form of in- (1) "not, opposite of" + politic (adj.) "judicious." Related: Impoliticly. Impolicy "quality of being impolitic" is attested from 1747.
imponderable (adj.): 1794, "weightless," from assimilated form of in- (1) "not, opposite of" + ponderable (see ponder). Figurative use, "unthinkable," from 1814. As a noun from 1829, originally meaning heat, light, electricity, etc., as having no weight. Related: Imponderably; imponderability. Imponderous is attested from 1640s as "without weight." Imponderabilia "unthinkable things collectively" is attested from 1835.
impedimenta (n.): "traveling equipment," c. 1600, from Latin impedimenta "luggage, military baggage," literally "hindrances," on the notion of "that by which one is impeded;" plural of impedimentum "hindrance" (see impediment).
impenitent (adj.): early 15c., from Latin impaenitentem, from assimilated form of in- "not, opposite of" (see in- (1)) + paenitens (see penitence). As a noun, "hardened sinner," from 1530s.
imperator (n.): "absolute ruler," 1580s, from Latin imperator "commander-in-chief, leader, master," agent noun from stem of imperare "to command" (see imperative (adj.)). In the Roman republic, a holder of military command during active service, also a title bestowed on victorious generals; in the Roman Empire, the emperor as commander-in-chief of the armies. Related: Imperatorial.
imperium (n.): "authority to command the national military forces," in extended use "an empire," 1650s, from Latin imperium "command, supreme authority, power" (see empire). Hence Latin phrase imperium in imperio "a state within a state."
impressment (n.): 1796, "act of impressing into public service or use," from impress (v.2) + -ment.
improv (n.): 1970 as colloquial shortening for improvisation. The famous New York City comedy club, founded in 1963, was, in full, The Improvisation.
imprompt (adj.): "not ready, unprepared," 1759, from Latin impromptus "unready, hesitating," from assimilated form of in- "not, opposite of" (see in- (1)) + promptus "ready" (see prompt (v.)).
impropriety (n.): 1610s, "quality or fact of being improper," from French impropriété (16c.) or directly from Latin improprietas "impropriety," from improprius "improper" (see improper). As "improper thing," 1670s.
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 Old French imprudence (14c.) or directly from Latin imprudentia "lack of foresight, inconsiderateness, ignorance, inadvertence," noun of quality from imprudens "unaware, inconsiderate" (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.
impulsion (n.): early 15c., "a driving, pushing, thrusting," from Old French impulsion (14c.), from Latin impulsionem (nominative impulsio) "external pressure," figuratively "incitement, instigation," noun of action from past participle stem of impellere (see impel).
impune (adj.): "unpunished" (obsolete), 1610s, from Latin impunis "unpunished" (see impunity). For the word meaning "attack by argument," see impugn.
Italianate (adj.): 1570s, from Italian Italianato "rendered Italian," from Italiano (see Italian). In older use "applied especially to fantastic affectations of fashions borrowed from Italy" [Century Dictionary], or in reference to the supposed Italian proverb that translates as an Englishman Italianate is a Devil incarnate which circulated in English (there also was a version in Germany about Italianized Germans).
investigate (v.): c. 1500, back-formation from investigation or else from Latin investigatus, past participle of investigare "to trace out, search after," figuratively "search into, investigate," from in- "in, into" (see in- (2)) + vestigare "to track, trace," from vestigium "footprint, track" (see vestige). Related: Investigated; investigating.
intro (n.): short for introduction, attested from 1923.
invade (v.): "enter in a hostile manner," late 15c., from Latin invadere "to go, come, or get into; enter violently, penetrate into as an enemy, assail, assault, make an attack on," from in- "in" (see in- (2)) + vadere "go, walk" (see vamoose). Compare evade. Related: invaded; invading.
inversion (n.): 1550s, "act of inverting;" 1590s, "state of being inverted," from Latin inversionem (nominative inversio) "an inversion," noun of action from past participle stem of invertere "turn about, turn upside-down" (see invert). Meteorological sense is from 1902. In old psychology, "homosexuality" (1895, short for sexual inversion) but in later psychology "identification with the opposite sex" (1958).
internationalism (n.): 1851, from international + -ism. Related: Internationalist.
interact (v.): "act on each other, act reciprocally," 1805, from inter- + act (v.). Related: Interacted; interacting.
infantile (adj.): mid-15c., "pertaining to infants," from Late Latin infantilis "pertaining to an infant," from infans "young child" (see infant). Sense of "infant-like" is from 1772.
infection (n.): late 14c., "infectious disease; contaminated condition;" from Old French infeccion "contamination, poisoning" (13c.) and directly from Late Latin infectionem (nominative infectio) "infection, contagion," noun of action from past participle stem of Latin inficere "to spoil, to stain" (see infect). Meaning "communication of disease by agency of air or water" (distinguished from contagion, which is body-to-body communication), is from 1540s.
infectious (adj.): "catching, having the quality of spreading from person to person, communicable by infection," 1540s of diseases, 1610s of emotions, actions, etc.; see infection + -ous. Earlier in the same sense were infectuous (late 15c.), infective (late 14c.). Related: Infectiously; infectiousness.
inferno (n.): 1834, "Hell, the infernal regions," from Italian inferno, from Late Latin infernus "Hell," in classical Latin "the lower world" (see infernal). As "a large, raging fire" from 1928.
informative (adj.): 1650s, "instructive, didactic," from Medieval Latin informativus, from Latin informatus, past participle of informare "to train, instruct, educate" (see inform). In Middle English, the same word meant "formative, shaping, plastic, having power to form or animate" (late 14c.). Related: Informatively.
incandescence (n.): 1650s, figurative, "state of being 'inflamed,' " from French incandescence, from incandescent (see incandescent). Literal use from 1794.
incept (v.): 1560s, "to commence, begin" (trans.), from Latin inceptus, past participle of incipere "to begin" (see inception). Related: Incepted; incepting.
inceptive (adj.): 1650s, "denoting the initial point or step," from Middle French inceptif (16c.), from Latin incept-, past participle stem of incipere "to begin" (see inception). Interchangeable with inchoative. As a noun, "an inceptive verb," from 1610s.
incestuous (adj.): 1530s, from Late Latin incestuosus "incestuous," from Latin incestus "unchaste" (see incest). Figurative use is from 1744. Related: Incestuously; incestuousness.
incertitude (n.): mid-15c., "variability," from Middle French incertitude (14c.), from Late Latin incertitudinem (nominative incertitudo) "uncertainty," from in- "not, opposite of" (see in- (1)) + certitudo "that which is certain" (see certitude). From c. 1600 as "doubt, hesitation." Middle English also had incertain "uncertain" and incertainty "uncertainty," both from Old French, but both have been displaced by forms in un-.
incidentally (adv.): 1520s, "by the way, casually;" see incidental + -ly (2). Sense of "as a new but related point" attested by 1925.
incidence (n.): early 15c., "incidental matter," from Middle French incidence (15c.), from Late Latin incidentia, from incidere "to happen, befall" (see incident (n.)). Meaning "act of coming into contact with or affecting" is from 1650s. In physics, of rays of light, etc., considered with reference to direction, from 1620s.
incise (v.): "to make a cut," 1540s, from French inciser (15c.), from Old French enciser "cut, cut out, slice" (12c.), from Latin incisus, past participle of incidere "to cut into, cut through" (see incision). In geology, of rivers, from 1893. Related: Incised; incising.
incitement (n.): 1590s, "action of inciting; that which incites," from French incitement (16c.), from Latin incitamentum, from incitare (see incite). Earlier was incitation (early 15c.).
inclusion (n.): c. 1600, "act of making a part of," from Latin inclusionem (nominative inclusio) "a shutting up, confinement," noun of action from past participle stem of includere (see include). Meaning "that which is included" is from 1839.
incognizant (adj.): also incognisant, 1826, from in- (1) "not, opposite of" + cognizant. Related: Incognizance.
incivility (n.): 1580s, "want of civilized behavior, rudeness;" 1610s, "uncourteous behavior to others," from French incivilité (15c.), from Late Latin incivilitatem (nominative incivilitas), from incivilis "not civil," from in- "not, opposite of" (see in- (1)) + civilis "relating to a citizen, relating to public life, befitting a citizen; popular, affable, courteous" (see civil). Meaning "an act of rudeness" is from 1650s. Incivil "not conducive to common good" is from mid-15c.
incommensurate (adj.): "not of equal measure; not having a common measure," 1640s, from in- (1) "not, opposite of" + commensurate.
incoherency (n.): "want of coherence in thought," 1680s, from incoherent + -cy.
incommodious (adj.): 1550s, from in- (1) "not, opposite of" + commodious. Related: Incommodiously. A verb, incommode, is attested from late 16c., from Latin incommodare. The Latin adjective was incommodus "inconvenient."
incompetence (n.): 1660s, "inadequacy;" 1716, "want of skill," from or modeled on French incompétence (16c.), from in- "not, opposite of, without" (see in- (1)) + compétence (see competence). Native formation incompetency is older (1610s).