Online Etymology Dictionary

ileus (n.): painful intestinal condition, 1706, from Latin ileus "severe colic," from Greek eileos "colic," from eilein "to turn, squeeze," from PIE *wel- (3) "to turn, roll" (see volvox).
incapacitation (n.): 1741, noun of action from incapacitate.
incarnate (adj.): late 14c., "embodied in flesh, in human or bodily form" (of souls, spirits, etc.), from Late Latin incarnatus "made flesh," a frequent word among early Christian writers, past-participle adjective from Latin incarnare "to make flesh" (see incarnation). Of qualities or abstractions, 1530s.
incautious (adj.): 1650s, from in- (1) "not" + cautious (adj.). The Latin adjective was incautus. Related: Incautiously. Incaution (n.) is attested from 1715.
inability (n.): mid-15c., inhabilite, "disqualification for office," from in- (1) + ability. Earlier was unability "incapability; incompetence" (late 14c.). General sense "state of being unable" is recorded by c. 1500.
inable (v.): obsolete form of enable.
inaccessible (adj.): early 15c., from Middle French inaccessible (14c.), from Late Latin inaccessibilis "unapproachable," from in- "not" (see in- (1)) + accessibilis "approachable" (see accessible). Related: Inaccessibly; inaccessibility. Earlier in same sense was unaccessible (c. 1400).
inaccuracy (n.): 1701, "quality or condition of being inaccurate," from inaccurate + -cy. As "an instance of being inaccurate, that which is inaccurate," 1704.
inaccurate (adj.): 1690s, from in- (1) "not" + accurate. Unaccurate is attested from 1670s. Related: Inaccurately (1660s).
inaction (n.): 1705, from in- (1) "not, opposite of" + action (n.). Perhaps modeled on French Inaction.
inactive (adj.): 1640s, from in- (1) "not, opposite of" + active. Perhaps a back-formation from Inactivity.
inactivity (n.): 1640s, from in- (1) "not, opposite of" + activity. Phrase masterly inactivity attested by 1791.
inadequacy (n.): 1764, from inadequate + -cy. Related: Inadequacies.
inadmissible (adj.): 1744, from in- (1) "not, opposite of" + admissible. Perhaps modeled on French inadmissible. Related: Inadmissibility.
inadvertence (n.): mid-15c., from Old French inadvertance "thoughtlessness, heedlessness" (14c.), from Scholastic Latin inadvertentia, from in- "not" (see in- (1)) + advertentia, from Latin advertere "to direct one's attention to," literally "to turn toward" (see advertise).
inadvisable (adj.): 1819, from in- (1) "not, opposite of" + advisable.
inalienable (adj.): "that cannot be given up," 1640s, from in- (1) "not, opposite of" + alienable (adj.). Perhaps from French inaliénable (16c.). Related: Inalienably; Inalienability.
inanimate (adj.): early 15c., "without vital force,having lost life," from Late Latin inanimatus "lifeless," from in- "not" (see in- (1)) + animatus (see animation). The Latin word closest corresponding in form and sense is inanimalis. Meaning "lacking vivacity, without spirit, dull" is from 1734. Inanimate as a verb meant "infuse with life or vigor" (17c.), from the other in- (see in- (2)).
inanity (n.): c. 1600, "emptiness, hollowness," literal and figurative, from French inanité (14c.) or directly from Latin inanitas "emptiness, empty space," figuratively "worthlessness," noun of quality from inanis "empty, void; worthless, useless," a word of uncertain origin. De Vaan writes that "The chronology of attestations suggests that 'empty, devoid of' is older than 'hollow'." Meaning "silliness, want of intelligence" is from 1753.
inapplicable (adj.): 1650s, from in- (1) "not, opposite of" + applicable. Related: Inapplicably; inapplicability.
inapposite (adj.): "not pertinent, not fit or suitable," 1620s (implied in inappositely), from in- (1) "not, opposite of" + apposite.
inapt (adj.): "ill-suited to the purpose or occasion," 1734, from in- (1) "not, opposite of" + apt. Related: Inaptly; inaptness. Compare inept.
inaptitude (n.): 1610s, from in- (1) "not, opposite of" + aptitude. The Frenchified version is ineptitude.
inattention (n.): 1710, from in- (1) "not, opposite of" + attention. Perhaps modeled on French inattention.
inattentive (adj.): 1650s, from in- (1) "not, opposite of" + attentive. Related: Inattentively; inattentiveness.
inaudible (adj.): c. 1600, "unable to be heard," from Late Latin inaudibilis "inaudible," from in- "not" (see in- (1)) + audibilis (see audible). Used in Middle English in the sense "unfit to be heard" (mid-15c.). Related: Inaudibly. Classical Latin had inauditus "unheard, unheard of."
inaugural (adj.): 1680s, from French inaugural (17c.), from inaugurer "to inaugurate" (14c.), from Latin inaugurare "to inaugurate" (see inauguration). The noun meaning "an inaugural address" is recorded from 1832, American English.
inaugurate (v.): "induct into office by formal ceremony," c. 1600, a back-formation from inauguration (q.v.) and also from Latin inauguratus, past participle of inaugurare. The etymological sense is "make a formal beginning or induction into office with suitable ceremonies" (which in ancient Rome included especially the taking of auguries). Related: Inaugurated; inaugurating; inaugurator.
inauspicious (adj.): 1590s, from in- (1) "not, opposite of" + auspicious. Related: Inauspiciously; inauspiciousness. The Latin word was inauspicatus "without auspices; with bad auspices," which had a brief career in English as inauspicate (17c.).
inauthentic (adj.): 1783, from in- (1) "not, opposite of" + authentic. Related: Inauthentically.
inbound (adj.): 1857, "homeward," from in + bound (adj.2). Originally of ships.
inbred (adj.): 1590s, "native, produced within," also "inherent by nature," from in + bred. The genetic sense is attested from 1892 (see inbreeding).
inbreeding (n.): "breeding of animals from the same parentage," c. 1842, from in + verbal noun from breed (v.). The older term for "to breed from animals of the same parentage" was to breed in and in (1765). The verb inbreed is attested from 1590s in the sense "produce within" (intransitive).
in-between (n.): 1815, "an interval;" also "a person who intervenes," noun use of prepositional phrase, from in (adv.) + between. Related: In-betweener (1912); in-betweenity (1927).
in vivo: 1898, Latin; "within a living organism," from vivere "to live" (see vital).
in vitro: 1892, scientific Latin; "in a test tube, culture dish, etc.;" literally "in glass," from Latin vitrum "glass" (see vitreous).
in utero: 1713, Latin, literally "in the uterus," from ablative of uterus (see uterus).
in toto (adv.): Latin, "as a whole, wholly, completely, utterly, entirely," from toto, ablative of totus "whole, entire" (see total (adj.)); "always or nearly always with verbs of negative sense" [Fowler].
in situ: 1740, Latin, literally "in its (original) place or position," from ablative of situs "site" (see site (n.)).
in memoriam: Latin, literally "in memory of," from accusative of memoria "memory" (see memory). The phrase was much-used in Latin writing; Tennyson's poem of that name (published in 1850) seems to have introduced the phrase to English.
in absentia (adv.): Latin, literally "in (his/her/their) absence" (see absence). By 1831 in English, earlier in legal Latin.
incrustation (n.): in early use also also encrustation, 1640s, from French encrustation, incrustation, from encruster (see encrust). Or perhaps from the obsolete English verb incrustate, which is attested from 1560s.
incumbency (n.): "state of being an incumbent," c. 1600, from incumbent + -cy.
incumbrance (n.): see encumbrance.
incunabula (n.): "swaddling clothes," also, figuratively, "childhood, beginnings, place where a thing had its earliest development," 1824, from Latin incunabula (neuter plural) "a cradle; a birthplace," figuratively "rudiments or beginnings," from in- "in" (see in- (2)) + cunabula, diminutive of cunae "cradle," from PIE *koi-na-, from root *kei- "to lie; bed, couch" (see cemetery).
incunabulum (n.): 1861, singular of Latin incunabula "cradle, birthplace; rudiments or beginnings" (see incunabula); taken up (originally in German) as a word for any book printed late 15c., in the infancy of the printer's art.
incurious (adj.): 1560s, "negligent, heedless," from Latin incuriosus "careless, negligent, unconcerned," from in- "not, opposite of, without" (see in- (1)) + curiosus (see curious). Meaning "uninquisitive" is from 1610s. Objective sense of "unworthy of attention" is from 1747. Related: Incuriously. Incuriosity is attested from c. 1600.
incus (n.): middle ear bone, 1660s, from Latin incus "anvil," from incudere "to forge with a hammer," from in- "in" + cudere "to strike, beat," from PIE *kau-do-, suffixed form of root *kau- "to hew, strike" (see hew). The bone so called by Belgian anatomist Andreas Vesalius (1514-1564).
indecency (n.): 1580s, "outrageous conduct," from Latin indecentia "unseemliness, impropriety," noun of quality from indecentem "unbecoming" (see indecent). Now especially of conduct which violates recognized standards of propriety (1690s).
indentured (adj.): "bound by indenture," 1748 (in indentured servant), past participle adjective from indenture (v.).