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.
illusion (n.): mid-14c., "mockery, scorning, derision;" late 14c., "act of deception; deceptive appearance, apparition; delusion of the mind," from Old French illusion "a mocking, deceit, deception" (12c.), from Latin illusionem (nominative illusio) "a mocking, jesting, jeering; irony," from past participle stem of illudere "mock at," literally "to play with," from assimilated form of in- "at, upon" (see in- (2)) + ludere "to play" (see ludicrous). Sense of "deceptive appearance" first developed in Church Latin. Related: Illusioned "full of illusions" (1920).
illusionary (adj.): 1811, from illusion + -ary.
illusionist (n.): "conjurer, magic act performer," 1840, from illusion + -ist. Earlier "one suffering from illusions" (1812). Middle English had illusor "deceiver, deluder."
illusive (adj.): "deceptive, false, illusory," 1670s, from stem of illusion + -ive. The older adjective is illusory.
illusory (adj.): 1590s, from French illusorie, from Late Latin illusorius "ironical, of a mocking character," from illus-, past participle stem of Latin illudere "mock, jeer at, make fun of," literally "play with," from assimilated form of in- "at, upon" (see in- (2)) + ludere "to play" (see ludicrous).
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.
illustrated (adj.): "provided with drawings, etc., as illustration," 1831, past participle adjective from illustrate (v.).
illustration (n.): c. 1400, "a shining;" early 15c., "a manifestation;" mid-15c., "a spiritual illumination," from Old French illustration "apparition, appearance" (13c.) and directly from Latin illustrationem (nominative illustratio) "vivid representation" (in writing), literally "an enlightening," from past participle stem of illustrare "light up, make light, illuminate;" figuratively "make clear, disclose, explain; adorn, render distinguished," from assimilated form of in- "in" (see in- (2)) + lustrare "make bright, illuminate," related to lucere "shine," lux "light" (see light (n.)). Mental sense of "act of making clear in the mind" is from 1580s. Meaning "an illustrative picture" is from 1816.
illustrative (adj.): 1640s, from illustrat-, past participle stem of Latin illustrare (see illustration) + -ive.
illustrator (n.): 1590s, "one who enlightens," agent noun in Latin form from illustrate, or from Late Latin illustrator "an enlightener." Meaning "one who draws pictures" is 1680s.
illustrious (adj.): 1560s, "distinguished by greatness, renowned," from Latin illustris "lighted, bright, brilliant;" figuratively "distinguished, famous," probably a back-formation from illustrare "make light, light up, illuminate," figuratively "embellish, distinguish, make famous" (see illustration). Replaced illustre in same sense (mid-15c.), from Middle French illustre.
illy (adv.): "in an ill manner," 1540s, from ill (adj.) + -ly (2). Correctly formed but seldom used; simple ill generally serving as the adverb.
Illyria: ancient name of the country on the east shore of the Adriatic, at its greatest extending inland to the Danube, a name of obscure origin. Later a name of a division of Austria-Hungary including Carinthia, Slovenia, and the coastal region around Istria. Related: Illyrian.
Ilocano: from Philippine Spanish Ilocos, literally "river men," from Tagalog ilog "river."
im-: variant of in- before -b-, -m-, -p- in the sense of "not, opposite of" (immobile, impersonal; see in- (2)) as well as "in, into" (implant, impoverish; see in- (1)). In some English words it alternates with em- (1).
image (n.): c. 1200, "piece of statuary; artificial representation that looks like a person or thing," from Old French image "image, likeness; figure, drawing, portrait; reflection; statue," earlier imagene (11c.), from Latin imaginem (nominative imago) "copy, imitation, likeness; statue, picture," also "phantom, ghost, apparition," figuratively "idea, appearance," from stem of imitari "to copy, imitate" (see imitation).
To þe ymage of god he made hym [Gen. i:27, Wycliffite Bible, early version, 1382]
Meaning "reflection in a mirror" is early 14c. The mental sense was in Latin, and appears in English late 14c. Sense of "public impression" is attested in isolated cases from 1908 but not in common use until its rise in the jargon of advertising and public relations, c. 1958.
image (v.): late 14c., "to form a mental picture (of something), imagine," from Old French imagier, from image (see image (n.)). Related: Imaged; imaging.
imagery (n.): mid-14c., "piece of sculpture, carved figures," from Old French imagerie "figure" (13c.), from image "likeness, figure, drawing, portrait" (see image (n.)). Rhetorical meaning "ornate description, exhibition of images to the mind" (in poetry, etc.) is from 1580s.
imaginable (adj.): late 14c., ymaginable, from Old French imaginable and directly from Late Latin imaginabilis, from Latin imaginari "picture to oneself" (see imagine). Related: Imaginably.
imaginary (adj.): "not real, existing only in fancy," late 14c., ymaginaire, from imagine + -ary; or else from Late Latin imaginarius "seeming, fancied," also literal, "pertaining to an image," from Latin imaginari "picture to oneself." Imaginary friend (one who does not exist) attested by 1789.
imagination (n.): "faculty of the mind which forms and manipulates images," mid-14c., ymaginacion, from Old French imaginacion "concept, mental picture; hallucination," from Latin imaginationem (nominative imaginatio) "imagination, a fancy," noun of action from past participle stem of imaginari (see imagine).
imaginative (adj.): late 14c., ymaginatyf, "pertaining to imagination; forming images, given to imagining," from Old French imaginatif and directly from Medieval Latin imaginativus, from imaginat-, stem of Latin imaginari "picture to oneself" (see imagine). Meaning "resulting from imagination" is from 1829. Related: Imaginatively; imaginativeness.
imagine (v.): mid-14c., "to form a mental image of," from Old French imaginer "sculpt, carve, paint; decorate, embellish" (13c.), from Latin imaginari "to form a mental picture, picture to oneself, imagine" (also, in Late Latin imaginare "to form an image of, represent"), from imago "an image, a likeness" (see image (n.)). Sense of "suppose, assume" is first recorded late 14c. Related: Imagined; imagining.
imagism (n.): name of a movement in poetry that sought clarity of expression through use of precise visual images, "hard light, clear edges" [Pound], coined 1912 by Ezra Pound; see image (n.) + -ism. Related: Imagist.
—Say it, no ideas but in things—
nothing but the blank faces of the houses
and cylindrical trees
bent, forked by preconception and accident—
split, furrowed, creased, mottled, stained—
secret—into the body of the light!

[William Carlos Williams, from "Paterson"]
imago (n.): "final or adult stage of an insect," 1797, from Latin imago "an image, a likeless" (see image (n.)). "The name is due to the fact that such an insect, having passed through its larval stages, and having, as it were, cast off its mask or disguise, has become a true representation or image of its species." [Century Dictionary]
imam (n.): 1610s, from Arabic, literally "leader; one who precedes," from amma "to go before, precede." As a high religious title used differently by Sunni and Shiite, but also used of the leader of daily prayers in the mosque and generally for a Muslim prince or religious leader. Related: Imamate.
imbalance (n.): 1895, from im- "not" + balance (n.).
imbecile (adj.): 1540s, imbecille "weak, feeble" (especially in reference to the body), from Middle French imbecile "weak, feeble" (15c.), from Latin imbecillus "weak, feeble" (see imbecility). Sense shifted to mental weakness or incapacity from mid-18c. (compare frail, which in provincial English also could mean "mentally weak"). As a noun, "feeble-minded person," it is attested from 1802. Traditionally an adult with a mental age of roughly 6 to 9 (above an idiot but beneath a moron).
imbecilic (adj.): 1875, from the noun imbecile + -ic.
imbecility (n.): early 15c., "physical weakness, feebleness (of a body part), impotence," from Middle French imbécillité and directly from Latin imbecillitatem (nominative imbecillitas) "weakness, feebleness, helplessness," from imbecillus "weak, feeble," of uncertain origin. "Weakness in mind" (as opposed to body) was a secondary sense in Latin but was not attested in English until 1620s.

The Latin word traditionally is said to mean "unsupported" or "without a walking stick" (Isidore: quasi sine baculo), from assimilated form of in- "not, opposite of" (see in- (1)) + baculum "a stick" (see bacillus), but Century Dictionary finds that "improbable" and de Vaan adds "it seems to me that exactly the persons who can walk without a support are the stronger ones."
imbibe (v.): late 14c., from Old French imbiber, embiber "to soak into," and directly from Latin imbibere "absorb, drink in, inhale," from assimilated form of in- "into, in, on, upon" (see in- (2)) + bibere "to drink," related to potare "to drink," from PIE *po(i)- "to drink" (see potion). Figurative sense of "mentally drink in" (knowledge, ideas, etc.) was the main one in classical Latin, first attested in English 1550s. Related: Imbibed; imbibing.
imbricate (v.): "to lay one over another" (as shingles, etc.), 1704 (implied in imbricated), from Latin imbricatus "covered with tiles," past participle of imbricare "to cover with rain tiles" (see imbrication). As an adjective from 1650s. Related: Imbricated; imbricating.
imbrication (n.): "an overlapping of edges" (as of roof tiles, etc.), 1640s, from French imbrication, noun of action from stem of Latin imbricare "to cover with tiles," from imbricem (nominative imbrex) "curved roof tile used to draw off rain," from imber (genitive imbris) "rain, heavy rain; rainwater," from PIE *ombh-ro- "rain" (source also of Sanskrit abhra "cloud, thunder-cloud, rainy weather," Greek ombros "rain, a shower"), from root *nebh- "moist; water" (see nebula).
imbroglio (n.): 1750, "a jumble;" 1818 as "complicated misunderstanding, intricate entanglement" (of persons, nations, etc.), from Italian imbroglio, from imbrogliare "confuse, tangle," from assimilated form of in- "into, in, on, upon" (see in- (2)) + brogliare "embroil," probably from Middle French brouiller "confuse" (see broil (v.2); also compare embroil).
imbrue (v.): early 15c., "to soak, steep;" mid-15c., "to stain, soil," from Old French embruer "to moisten," which probably is a metathesis of embevrer "give to drink, make drunk," from em- (see em-) + -bevrer, ultimately from Latin bibere "to drink" (see imbibe). Or perhaps from Old French embroue "soiled," ultimately from boue "mud, dirt."
imbue (v.): early 15c., "to keep wet; to soak, saturate;" also figuratively "to cause to absorb" (feelings, opinions, etc.), from Latin imbuere "moisten, wet, soak, saturate," figuratively "to fill; to taint," a word of uncertain origin, perhaps from the same root as imbrication. Compare also Old French embu, past participle of emboivre, from Latin imbibere "drink in, soak in" (see imbibe), which might have influenced the English word. Related: Imbued; imbuing.
imburse (v.): "supply with money, store up," literally "put in a purse," 1520s, from Medieval Latin imbursare, from assimilated form of in- "into, in, on, upon" (see in- (2)) + bursa "purse" (see bursa). Related: Imbursement.
imitable (adj.): 1550s, from French imitable (16c.), from Latin imitabilis "that may be imitated," from imitari "to copy, portray" (see imitation). Related: Imitability.
imitate (v.): 1530s, a back-formation from imitation or imitator, or else from Latin imitatus, past participle of imitari "to copy, portray." Related: Imitated; imitating. An Old English word for this was æfterhyrigan.
imitation (n.): c. 1400, "emulation; act of copying," from Old French imitacion, from Latin imitationem (nominative imitatio) "a copying, imitation," noun of action from past participle stem of imitari "to copy, portray, imitate," from PIE *im-eto-, from root *aim- "copy" (source also of Hittite himma- "imitation, substitute;" Latin imago "image," aemulus "emulous"). Meaning "an artificial likeness" is from c. 1600. As an adjective, from 1840.
imitative (adj.): 1580s, probably from imitate + -ive; or else from Middle French imitatif, from Late Latin imitativus, from imitat-, stem of Latin imitari "to copy, portray" (see imitation).
imitator (n.): 1520s, from Middle French imitateur (14c.) or directly from Latin imitator "a copyist; a mimic," from imitari "to copy, imitate" (see imitation).
immaculacy (n.): "state of being immaculate," 1785; see immaculate + -cy. Earlier nouns were immaculateness (1640s), immaculation (c. 1600).
immaculate (adj.): mid-15c., "free from mental or moral pollution, pure," from a figurative use of Latin immaculatus "unstained," from assimilated form of in- "not, opposite of" (see in- (1)) + maculatus "spotted, defiled," past participle of maculare "to spot," from macula "spot, blemish," a word of uncertain origin. The literal sense of "spotlessly clean or neat" in English is first attested 1735. Related: Immaculately.

The phrase Immaculate Conception "freedom from original sin possessed by the Virgin Mary from her conception in her mother's womb" is from late 15c. in English (from Middle French conception immaculée); the idea itself had been debated in the Church since 12c., declared to be an article of faith in 1854.
immanence (n.): "fact or state of indwelling," 1816; see immanent + -ence. Immanency is from 1650s.
immanent (adj.): "indwelling, remaining within, inherent," 1530s, via French immanent (14c.) or directly from Late Latin immanens, present participle of immanere "to dwell in, remain in," from assimilated form of in- "into, in, on, upon" (see in- (2)) + Latin manere "to dwell" (see manor). In medieval philosophy contrasted with transitive; later with transcendent. Related: Immanently.
Immanuel: masc. proper name, literally "God with us;" see Emmanuel.
immarcescible (adj.): also immarcessible (but this is considered less correct), "unfading, imperishable," early 15c., from Late Latin immarcescabilis from assimilated form of in- "not" (see in- (1)) + stem of Latin marcescere "to begin to wither, grow feeble, pine away" (see marcescent).
immaterial (adj.): c. 1400, "spiritual, incorporeal, not consisting of matter," from Medieval Latin immaterialis "not consisting of matter, spiritual," from assimilated form of in- "not, opposite of" (see in- (1)) + Late Latin materialis "of or belonging to matter" (see material (adj.)). Sense of "unimportant, of no consequence" is first recorded 1690s from material (adj.) in its meaning "important" (16c.). Related: Immaterially (late 14c.); immateriality.