Online Etymology Dictionary

illegality (n.): quality of being illegal," 1630s, from illegal (adj.) + -ity; or else from French illegalité (14c.).
illegally (adv.): 1620s, from illegal (adj.) + -ly (2).
illegible (adj.): 1630s, from assimilated form of in- (1) "not, opposite of" + legible. Related: Illegibly; illegibility.
illegitimacy (n.): 1670s; see illegitimate + -acy.
illiquid (adj.): 1690s, from assimilated form of in- (1) "not" + liquid (adj.) in the financial sense.
illiteracy (n.): 1650s, "inability to read and write," from illiterate + -cy. Earlier in this sense was illiterature (1590s).
illimitable (adj.): 1590s, from assimilated form of in- (1) "not" + limitable.
illiberal (adj.): 1530s, "ungentlemanly, base, mean," from Middle French illiberal (14c.), from Latin illiberalis "ungenerous, mean, sordid; unworthy of a freeman; stingy, disobliging," from assimilated form of in- "not, opposite of" (see in- (1)) + liberalis (see liberal (adj.)). A sense of "narrow-minded politically; unconcerned with the rights or liberties of others" is attested from 1640s (as a noun in this sense 1818), and might be revived to ease the load of meanings that weighs on conservative.
illogical (adj.): "without sound reasoning according to rules of logic," 1580s, from assimilated form of in- (1) "not, opposite of" + logical. Related: Illogically.
Ilocano: from Philippine Spanish Ilocos, literally "river men," from Tagalog ilog "river."
impaction (n.): 1739, from Latin impactionem (nominative impactio) "a striking against," noun of action from past participle stem of impingere "drive into, strike against" (see impinge).
impalpable (adj.): c. 1500, "too unsubstantial to be perceived by touch," from French impalpable or directly from Medieval Latin impalpabilis, from assimilated form of in- "not, opposite of" (see in- (1)) + palpabilis (see palpable). Figurative (mental) sense of "that cannot be grasped by the intellect" is from 1774. Related: Impalpably; impalpability.
impassable (adj.): "that cannot be passed or passed over," 1560s, from assimilated form of in- (1) "not, opposite of" + passable. Related: Impassability.
impassible (adj.): "incapable of feeling pain, exempt from suffering," mid-14c., from Old French impassible (13c.) or directly from Church Latin impassibilis "incapable of passion," from assimilated form of in- "not, opposite of" (see in- (1)) + passibilis "capable of passion, feeling, or suffering, from passio "suffering" (see passion). Meaning "emotionless" is from 1590s. Related: Impassibility.
impasto (n.): "laying on of colors thickly and boldly," 1784, from Italian impasto, noun of action from impastare "to raise paste; to put in paste," from assimilated form of in- "into, in" (see in- (2)) + pasta "paste" (see pasta). Nativized form impaste is attested from 1540s as "enclose in paste," 1727 in reference to painting. Related: Impastoed; impastation.
impedance (n.): "hindrance," especially and originally "resistance due to induction in an electrical circuit," 1886, from impede + -ance. The classically correct formation would be *impedience.
illude (v.): early 15c., to trick, deceive; treat with scorn or mockery," from Latin illudere "to make sport of, scoff at, mock, jeer at," from assimilated form of in- "in, into" (see in- (2)) + ludere "to play" (see ludicrous).
ikebana (n.): Japanese are of formal flower arrangement, 1901, from Japanese, from ikeru "to keep alive, arrange" + hana "flower."
idleness (n.): Old English idelnes "frivolity, vanity, emptiness; vain existence;" see idle (adj.) + -ness. Old English expressed the idea we attach to in vain by in idelnisse. In late Old English it began to acquire its sense of "state of being unoccupied, doing no work, or indolent." Similar formation in Old Saxon idilnusse, Old Frisian idlenisse, Old High German italnissa. Spenser, Scott, and others use idlesse to mean "condition of being idle" in a positive sense, as a pleasure.
idler (n.): "one who spends his time in inaction," 1530s, agent noun from idle (v.).
Ido: 1908, artificial language devised in 1907 and based on Esperanto; from Ido -ido "offspring," suffix representing Latin -ida, Greek -ides.
-ie: alternative spelling of -y; now mostly of -y (3), but formerly of the others as well.
illocutionary (adj.): 1955, from illocution + -ary.
illusionary (adj.): 1811, from illusion + -ary.
ideological (adj.): 1797, from ideology + -ical or from ideologic (from French idéologique) + -al (2). Related: Ideologically.
identifiable (adj.): 1804, from identify + -able. Related: Identifiably.
ideate (v.): c. 1600, "imagine, form ideas," from idea + -ate (2). From 1862 as "to think." Related: Ideated; ideating.
ideation (n.): 1829; see idea + -ation. Related: Ideational.
As we say Sensation, we might say also, Ideation; it would be a very useful word; and there is no objection to it, except the pedantic habit of decrying a new term. [James Mill, "Analysis of the Phenomena of the Human Mind," London, 1829]
icy (adj.): Old English isig; see ice (n.) + -y (2). Modern use is said to be a late Middle English re-formation. Figurative sense "characterized by coldness or chill, frigid" (of manners, expressions, etc.) is from 1590s. Similar formation in Dutch ijzig, German eisig, Swedish isig. Related: Icily; iciness.
iconic (adj.): 1650s, "of or pertaining to a portrait," from Late Latin iconicus, from Greek eikonikos "pertaining to an image," from eikon "likeness, image, portrait" (see icon). In art, applied to statues of victorious athletes, sovereigns, etc., 1801.
icosahedron (n.): "twenty-sided body," 1560s, from Latinized form of Greek eikosahedron, noun use of neuter of eikosahedros, from eikosi "twenty" + -hedra "seat, base, chair, face of a geometric solid," from PIE root *sed- (1) "to sit" (see sedentary). Greek eikosi is from PIE *wikmti- "twenty," from *wi- "in half" (hence "two") + (d)kmti-, from root *dekm- "ten" (see ten). Related: icosahedral.
icteric (adj.): "jaundiced," c. 1600, from Latin ictericus, from Greek ikterikos "jaundiced," from ikteros "jaundice" (see icterus). Related: Icterical.
icterus (n.): "jaundice," 1706, medical Latin, from Greek ikteros "jaundice," also the name of a yellowish bird the sight of which was supposed, by sympathetic magic, to cure jaundice (but the bird died). As a zoological genus (American orioles), from 1713.
ictus (n.): rhythmical or metrical stress, 1752, from Latin ictus "a blow, stroke, thrust;" of voices "a beat, impulse, stress," from icere (past participle ictus) "to strike, hit," which is related to iacere "to throw" (see jet (v.)). Reckoned in Anglo-Saxon poetry; in Modern English it generally is identical to syllabic stress or accent.
iconography (n.): 1670s, "illustration by drawing or figures," from Medieval Latin iconographia, from Greek eikonographia "sketch, description," from eikon (see icon) + -graphia (see -graphy). Related: Iconographic; iconographer.
iamb (n.): 1842, from French iambe (16c.) or directly from Latin iambus "an iambic foot; an iambic poem," from Greek iambos "metrical foot of one unaccented followed by one accented syllable" (see iambic). Iambus itself was used in English in this sense from 1580s.
iatrogenic (adj.): "induced by a physician," 1920, from iatro- + -genic.
Icarus: son of Daedalus in Greek mythology, from Latinized form of Greek Ikaros, a name of unknown origin, connected to Icaria and the Icarian Sea. He flew too high on artificial wings and so plunged to his death. Used allusively in English from 1580s.
ichthyosis (n.): 1815, coined in Modern Latin (1801); see ichthyo- + -osis. So called for the scales which form.
idiocrasy (n.): "peculiarity" (physical or mental), 1680s, from Latinized form of Greek idiokrasia, from idios "one's own, personal" (see idio-) + krasis "mixing, tempering," from PIE root *kere- "to mix, confuse; cook" (see rare (adj.2)). Related: Idiocratic.
idiocy (n.): "state of being an idiot," 1520s, from idiot on model of prophecy, etc. Early alternatives included idiotacy (1580s), idiotry (1590s).
ideologue (n.): 1815, in reference to the French Revolutionaries, from French ideologue, from Greek idea (see idea) + -logos (see -logue). Earlier form was ideologist (1798).
ideograph (n.): "character or symbol which suggests an object without expressing its name," 1841, from ideo-, here as a comb. form of idea, + -graph "instrument for recording; something written." Related: Ideographic (1822); ideographical.
ideogram (n.): "ideograph," 1837, from ideo-, here as a comb. form of idea, + -gram.
idempotent (n.): in algebra, quantity which multiplied by itself gives itself, 1870, from Latin idem "the same, identical with" (see idem) + potentem "powerful" (see potent).
idiopathy (n.): "primary disease," 1690s, Modern Latin, from medical Greek idiopatheia, from idios "one's own" (see idiom) + -patheia, abstract noun formation from pathos "suffering, disease, feeling" (see pathos). Related: idiopathic.
idiolect (n.): one's personal way of using a language, 1948, from idio- "one's own, personal" + second element abstracted from dialect. Idioglottic (1888) has a sense "using words invented in one's mind" (from Greek glotta/glossa "tongue").
imperceptible (adj.): early 15c., from Middle French imperceptible (15c.), from Medieval Latin imperceptibilis, from assimilated form of in- "not, opposite of" (see in- (1)) + perceptibilis (see perceptible). Related: Imperceptibly; imperceptibility. OED marks imperceivable (1610s) as "Now rare."
imperforate (adj.): "having no perforation," 1670s, from assimilated form of in- (1) "not, opposite of" + perforate (adj.). Related: Imperforation (1650s).
imperil (v.): 1590s, from assimilated form of in- "into, in" (see in- (2)) + peril. Formerly also emperil. Related: Imperiled; imperiling; imperilment.