Online Etymology Dictionary

illumine (v.): late 14c., "to enlighten spiritually;" mid-15c., "to light up, shine light on," from Old French illuminer (13c.), from Latin illuminare "make bright, light up" (see illumination). Related: illumined.
illustrate (v.): 1520s, "light up, shed light on;" 1610s, "educate by means of examples," back-formation from illustration, and in some cases from Latin illustratus, past participle of illustrare "light up, make light, illuminate." Sense of "provide pictures to explain or decorate" is 1630s. Related: Illustrated; illustrating.
illness (n.): "disease, sickness, ailment, malady," 1680s, from ill (adj.) + -ness. Earlier it meant "bad moral quality" (c. 1500).
-ible: word-forming element making adjectives from verbs, borrowed in Middle English from Old French -ible and directly from Latin adjective suffix -ibilis (properly -bilis); see -able.
ichthyology (n.): 1640s, from ichthyo- "fish" + -ology. Related: Ichthyologist; ichthyological.
-ide: word-forming element used to coin names for simple compounds of one element with another element or radical; originally abstracted from oxide, which was the first so classified.
idealize (v.): 1786, "make ideal, consider as ideal," probably formed from ideal (adj.) + -ize. Related: Idealized; idealizing.
identification (n.): 1640s, "treating of a thing as the same as another; act of making or proving to be the same," from French identification, probably from identifier (see identify). Psychological sense of "becoming or feeling oneself one with another" is from 1857. Meaning "act or process of determining the identity of something" is from 1859. Meaning "object or document which marks identity" is from 1947 (short for identification tag, card, etc.).
ice-cream (n.): 1744, earlier iced cream (1680s), from ice (n.) + cream (n.).
icing (n.): 1769 in the confectionery sense, "coating of concreted sugar," verbal noun of ice (v.). Earlier in this sense was simple ice (1723); frosting came later. Meaning "process of becoming covered with ice" is from 1881.
inestimable (adj.): late 14c., "beyond estimation or measure, not to be computed," from Old French inestimable "priceless" (14c.) or directly from Latin inaestimabilis "invaluable, incalculable," also "not estimable, valueless," from in- "not, opposite of" (see in- (1)) + aestimabilis "valuable, estimable," from aestimare (see esteem (v.)). Meaning "too precious to set a value on, priceless" is attested by 1570s. Related: Inestimably; inestimability.
ineradicable (adj.): 1794, from in- (1) "not, opposite of" + eradicable (see eradicate). Related: Ineradicably.
inescapable (adj.): 1792, from in- (1) "not, opposite of" + escapable (see escape (v.)). Related: Inescapably.
ineptitude (n.): 1610s, from French ineptitude, from Latin ineptitudo, noun of quality from ineptus "unsuitable, absurd" (see inept).
indwelling (n.): "act of residing," late 14c. (Wyclif's translation of Latin inhabitatio), present participle of obsolete indwell, from in (adv.) + dwell (v.). He also used indweller for Latin inhabitans and indwell (v.) for inhabitare.
industrialization (n.): 1883, noun of action from industrialize (q.v.).
industrialist (n.): 1846, from industrial + -ist. Perhaps modeled on French industrialiste (Saint-Simon, 1823). Earlier "one who makes a living by productive industry" (1837).
industrialism (n.): 1831, from industrial + -ism. Probably modeled on French industrialisme (Saint-Simon, 1823).
industrialisation (n.): chiefly British English spelling of industrialization (q.v.); for spelling, see -ize.
individually (adv.): 1590s, "indivisibly," from individual + -ly (2). Meaning "as individuals" is from 1640s.
indorsement (n.): see endorsement.
inducement (n.): 1590s, "that which induces," from induce + -ment.
inducive (adj.): "tending to induce," 1610s, from induce + -ive.
inductance (n.): 1879, in electricity, from induct + -ance.
induration (n.): late 14c., "a hardening or congealing" (of body parts, alchemical materials), from Old French induracion "hardness, obstinacy" (14c.) or directly from Medieval Latin indurationem (nominative induratio) "hardness (especially of the heart)," noun of action from past participle stem of Latin indurare "to make hard, harden" (see endure).
indurate (v.): 1590s (transitive) "make hard;" 1620s (intransitive) "grow harder," from Latin induratus, past participle of indurare "to make hard, harden" (see endure). Related: Indurated.
inductor (n.): 1650s, "one who initiates," agent noun from Latin stem of induce. Classical Latin inductor meant "one who stirs up, an instigator." Electromagnetic senses are from 1837.
inductive (adj.): early 15c., "bringing on, inducing," from Old French inductif or directly from Late Latin inductivus "serving to induce or infer," from induct-, past participle stem of Latin inducere (see induce). As a term in logic, "based on induction" (q.v.), from 1764. Related: Inductively.
individualist (n.): 1839, "egoist, free-thinker," from individual + -ist, and compare individualism. Related: Individualistic.
indite (v.): formerly also endite, late 14c., "put down in writing," from Old French enditer, enditier "dictate, write; draw up, draft; (legally) indict," from Vulgar Latin *indictare, from Latin in- "in, into, on, upon" (see in- (2)) + dictare "to declare" (see dictate (v.)). The same word as indict but retaining a French form. Related: Indited; inditing.
inditement (n.): 1560s, "action of writing prose or verse," from indite + -ment. Perhaps modeled on French enditement (12c.).
infrasonic (adj.): also infra-sonic, 1920, on the model of supersonic, etc., from infra- + sonic. Or perhaps modeled on French infra-sonore.
infringement (n.): "a break or breach" (of a contract, right, etc.), from infringe + -ment. Earlier in a now-obsolete sense of "contradiction" (1590s).
infotainment (n.): 1983, from info- + entertainment.
infusion (n.): c. 1400, "a liquid extract (obtained by soaking in water);" early 15c., "a pouring in; that which is poured in," from Old French infusion "injection" (13c.) or directly from Latin infusionem (nominative infusio) "a pouring in, a watering," noun of action from past participle stem of infundere "to pour into" (see infuse).
infundibulum (n.): 1799, "funnel-shaped organ or body part," from a Modern Latin use of Latin infundibulum "a funnel," from infundere "to pour into" (see infuse) + -bulum, suffix forming names of instruments. In some cases a loan-translation into Latin of Greek khoane "funnel." Related: Infundibular.
informant (n.): 1690s, "someone who supplies information," from Latin informantem (nominative informans), present participle of informare "train, instruct, educate" (see inform). Occasionally as "one who gives information to the authorities, one who dishonorably betrays knowledge gained in confidence" (1783). Informer is older in both senses and more usual in the latter. As an adjective from 1890.
infomercial (n.): 1983, from info- + commercial (n.). Before the televised infomercial was the newspaper advertorial (1961).
inhabitant (n.): "one who dwells in a place" (as distinguished from a visitor or transient), early 15c., from Anglo-French inhabitant, from Latin inhabitantem (nominative inhabitans), present participle of inhabitare "to dwell in" (see inhabit). Related: Inhabitants. As an adjective, also from early 15c.
inhabitable (adj.): 1. "not habitable," late 14c., from Old French inhabitable (14c.), from Latin inhabitabilis, from in- "not, opposite of" (see in- (1)) + habitabilis (see habitable).

2. "capable of being inhabited" (the main modern sense), c. 1600, from inhabit + -able). In Late Latin, inhabitabilis also was used in a sense of "that can be inhabited." A word used in two opposite senses.
injection (n.): "a forcing of a fluid into a body" (with a syringe, etc.), early 15c., from Old French iniection (14c.) or directly from Latin iniectionem (nominative iniectio) "a throwing in," noun of action from past participle stem of inicere "to throw in or on" (see inject).
insecticide (n.): "substance which kills insects," 1866 (from 1865 as an adjective), from insect + -cide. Earlier as a type of machine (1856). Related: Insecticidal (1857).
intensity (n.): 1660s, from intense + -ity. Earlier was intenseness (1610s). A scientific term originally; sense of "extreme depth of feeling" attested by 1830.
invalidate (v.): 1640s, from invalid (adj.2) + -ate (2). Related: Invalidated; invalidating.
irksome (adj.): "bothersome, troublesome, annoying," early 15c., from irk + -some (1). Related: Irksomely; irksomeness.
Indus: river in Asia, from Sanskrit sindhu "river." The southern constellation, created 1603 by Bayer, represents "an Indian," not the river.
inhumanity (n.): "barbarous cruelty," late 15c., from French inhumanité (14c.) or directly from Latin inhumanitatem (nominative inhumanitas) "inhuman conduct, savageness; incivility, rudeness," noun of quality from inhumanus "inhuman, savage, cruel" (see inhuman).
And Man, whose heav'n-erected face
The smiles of love adorn,--
Man's inhumanity to man
Makes countless thousands mourn!
[Robert Burns, "Man was Made to Mourn," 1784]
iniquitous (adj.): "unjust wicked," 1670s, from iniquity + -ous. Earlier were iniquous (1650s, from Latin iniquus) and inique (1520s, from French inique). Related: Iniquitously; iniquitousness.
initialize (v.): "to make ready for operation," 1957, from initial (adj.) + -ize. The same formation had been used earlier to mean "use initials instead of a name" (1837); "designate by initials" (1833). Related: Initialized; initializing; initialization (1957 in the modern sense).
injurious (adj.): early 15c., "abusive," from Old French injurios "unjust; harmful" (14c., Modern French injurieux) and directly from Latin iniuriosus "unlawful, acting unjustly, wrongful, harmful," from iniuria "injustice, unlawful violence, insult" (see injury). Related: Injuriously; injuriousness.