Online Etymology Dictionary

invigilate (v.): "to watch diligently" (archaic), 1550s, from Latin invigilatus, past participle of invigilare "watch over, be watchful, be devoted," from in- "in" (see in- (2)) + vigilare "to watch, keep awake, not sleep" (see vigil). In late 19c. especially in reference to student exams. Related: Invigilated; invigilating.
involution (n.): late 14c., "condition of being twisted or coiled; a fold or entanglement," originally in anatomy, from Late Latin involutionem (nominative involutio) "a rolling up," noun of action from past participle stem of Latin involvere "envelop, surround, roll into" (see involve). Related: Involutional.
involute (adj.): early 15c., "wrapped," from Latin involutus "rolled up, intricate, obscure," past participle of involvere "envelop, surround; roll into, wrap up" (see involve).
Io: in Greek mythology, daughter of the river god Inachus, she was pursued by Zeus, who changed her to a heifer in a bid to escape the notice of Juno, but she was tormented by a gadfly sent by Juno. The Jovian moon was discovered in 1610 and named for her by Galileo.
Ionic (adj.): "pertaining to Ionia or the Ionians," 1570s of music; 1580s of architecture, from Latin Ionicus, from Greek Ionikos (see Ionian). In prosody, a foot of two long syllables followed by two short. The Ionic school of philosophers (Thales, Anaxamander, etc.) studied the material world in ways that somewhat anticipated observational science. It also once was the name of an important school of Greek painting, but all of it save the name is lost. Related: Ionicize (1841).
ionization (n.): 1891; see ionize + -ation.
ionize (v.): 1896, from ion + -ize. Related: Ionized; ionizing. Unrelated to Ionize "to make Ionic in form or fact" (1816), for which see Ionian.
ionosphere (n.): region of the outer atmosphere, 1926, from ion + sphere. Coined by Scottish radar pioneer Robert A. Watson-Watt (1892-1973). So called because it contains many ions.
ipseity (n.): "personal identity, individuality, selfhood," 1650s, from Latin ipse "self" + -ity.
ipsilateral (adj.): "on the same side of the body," 1907, from Latin ipse "self" + lateral (adj.). Related: Ipsilaterally.
iridescence (n.): 1799, from iridescent + -ence. Related: Iridescency (1799).
intron (n.): 1978 in genetics, from intragenic "occurring within a gene" + -on.
ir-: assimilated form of the two Latin prefixes in- "not," or "in" (see in-) before -r-.
irradiance (n.): 1660s, from Latin irradiantem (nominative irradians), present participle of irradiare "to shine forth" (see irradiate). Related: Irradiancy (1640s).
irradiation (n.): 1580s, in reference to light (literally and figuratively), from French irradiation, noun of action from past participle stem of Latin irradiare (see irradiate). Of X-rays, etc., from 1901.
irrecoverable (adj.): mid-15c., from Old French irrecovrable (Modern French irrecouvrable), from assimilated form of in- "not, opposite of" (see in- (1)) + recovrable (see recover). In same sense irrecuperable (from Late Latin irrecuperabilis) is from mid-14c. Related: Irrecoverably.
irredeemable (adj.): c. 1600, from assimilated form of in- (1) "not, opposite of" + redeemable. Related: Irredeemably.
irreducible (adj.): 1530s, from assimilated form of in- (1) "not, opposite of" + reducible. Related: Irreducibly; irreducibility.
irregularity (n.): early 14c., "violation of Church rules governing admission to clerical office," from Old French irregularité (14c.) and directly from Medieval Latin irregularitas "irregularity," from irregularis "not regular" (see irregular (adj.)). Meaning "that which is irregular" is from late 15c.; sense of "state of non-conformity to rule" is from 1590s; meaning "want of symmetry" is from 1640s.
irrelevance (n.): 1735, from irrelevant + -ance. Earlier in the same sense was irrelevancy (1590s).
irremediable (adj.): "beyond remedy," mid-15c., from Late Latin irremediabilis "incurable," from assimilated form of in- "not, opposite of" (see in- (1)) + remediabilis "that may be healed, curable" (see remediable).
irreconcilable (adj.): 1590s, from assimilated form of in- (1) "not, opposite of" + reconcilable, if that word is so old. Or perhaps from French irréconcilable (16c.). Related: Irreconcilably. As a noun, "one who refuses reconciliation or compromise" (especially in politics), from 1748.
irreplaceable (adj.): 1806, from assimilated form of in- (1) "not, opposite of" + replaceable. Related: Irreplaceably.
irreprehensible (adj.): "blameless," late 14c., from Late Latin irreprehensibilis, from Latin irreprehensus "blameless, without blame," from assimilated form of in- "not, opposite of" (see in- (1)) + reprehensibilis, from past participle stem of Latin reprehendere "to blame, censure, rebuke; seize, restrain" (see reprehend).
irrespective (adj.): 1620s (implied in irrespectively), "disrespectful," from assimilated form of in- (1) "not, opposite of" + respective in its sense of "regardful." Meaning "without taking account of particular circumstances or conditions" had developed by 1690s, from the notion of "not observing or noting with attention." In modern use it tends to be adverbial, in irrespective of, a use attested by c. 1800.
isochronous (adj.): "uniform in time, of equal time, performed in equal times," 1706, with suffix -ous, from Modern Latin isochronus, from Greek isokhronos "equal in age or time," from iso- "equal" (see iso-) + khronos "time" (see chrono-). Earlier in same sense was isochronal (1670s).
isokinetic (adj.): 1942, from iso- + kinetic.
iso-: before vowels often is-, word-forming element meaning "equal, similar, identical; isometric," from Greek isos "equal to, the same as; equally divided; fair, impartial (of persons); even, level (of ground)," as in isometor "like one's mother." In English used properly only with words of Greek origin; the Latin equivalent is equi- (see equi-).
islet (n.): 1530s, from Middle French islette (Modern French îlette), diminutive of isle (see isle).
ischemia (n.): also ischaemia, 1866 (but as far back as 1660s in form ischaimes), from medical Latin ischaemia, from ischaemus "stopping blood," from Greek iskhaimos "stanching or stopping blood," from iskhein "to hold, curb, keep back, restrain" (from PIE *si-sgh-, reduplication of root *segh- "to hold" (see scheme (n.)) + haima "blood" (see -emia). Related: Ischemic.
isagoge (n.): 1650s, from Latin isagoge, from Greek eisagoge "an introduction (into court), importation (of goods)," from eis "into" + agoge "a leading," from agein "to lead" (see act). Related: Isagogic; isagogical (1520s); Isagogics.
irruption (n.): 1570s, from Middle French irruption (14c.) or directly from Latin irruptionem (nominative irruptio) "a breaking in, bursting in, invasion," noun of action from past participle stem of irrumpere "to break in, force one's way in, burst into," from assimilated form of in- "into, in, on, upon" (see in- (2)) + rumpere (see rupture (n.)). Frequently confused with eruption.
irrupt (v.): "to break into," 1805 (implied in irrupted), back-formation from irruption or else from Latin irruptus, past participle of irrumpere "to break in, burst into."
irritation (n.): early 15c., in physiology, in reference to sores and morbid swelling, from Middle French irritation or directly from Latin irritationem (nominative irritatio) "incitement, stimulus; irritation, wrath, anger," noun of action from past participle stem of irritare "to excite, provoke" (see irritate). Meaning "impatient or angry excitement" is from 1703.
irritant (adj.): 1630s, from Latin irritantem (nominative irritans), present participle of irritare "to excite, provoke" (see irritate). As a noun, "that which irritates," from 1802.
irritable (adj.): 1660s, "susceptible to mental irritation," from French irritable and directly from Latin irritabilis "easily excited," from irritare "excite, provoke" (see irritate). Meaning "responding quickly to a stimulus" is from 1791. Related: Irritably.
irrigation (n.): 1610s, "a supplying of water to land," also in medical use, "supply of a liquid to some part of the body," from Latin irrigationem (nominative irrigatio) "a watering, irrigation," noun of action from past participle stem of irrigare "lead water to, irrigate, flood" (see irrigate).
irreversible (adj.): 1620s, of decrees, etc., "that cannot be overturned or undone," from assimilated form of in- (1) "not, opposite of" + reversible. Of physical things, "that cannot be turned the other way," from 1821. Related: Irreversibly.
irretrievable (adj.): "not recoverable," 1690s (implied in irretrievably), from assimilated form of in- (1) "not, opposite of" + retrievable. Related: Irretrievability.
irresolute (adj.): "not firm in purpose, wavering, given to doubt or hesitation," 1570s, from Latin irresolutus "not loosed, not loosened," from assimilated form of in- "not, opposite of" (see in- (1)) + resolutus (see resolute). Related: Irresolutely.
irreputable (adj.): "disreputable," 1709, from assimilated form of in- (1) "not, opposite of" + reputable.
irreproachable (adj.): 1630s, from French irréprochable (15c.), from assimilated form of in- "not, opposite of" (see in- (1)) + réprochable (see reproach (n.)). Related: Irreproachably.
IRS: also I.R.S., initialism (acronym) of Internal Revenue Service, U.S. federal government tax collection agency, attested by 1954. The office dates to 1862; name changed 1953 from Bureau of Internal Revenue.
inbox (n.): by 1984 in electronic mail sense, from in + mailbox (n.). Compare in-basket, in reference to office mail systems, by 1940; in-tray (1917).
isomer (n.): 1852, in chemistry, back-formation from isomeric. A compound identical or nearly so in composition and molecular weight with another, but having different properties.
itchy (adj.): Old English giccig; see itch + -y (2). Figurative itchy palm is attested by 1599 (Jonson; Shakespeare has itching palm in the same sense, 1601). Other figurative uses include itching ears "a hankering for gossip," itching elbows "a passion for gambling." Related: Itchiness.
italicize (v.): "to print in italics" (for emphasis, etc.), 1795, from italic + -ize. Related: Italicized; italicizing; italicization.
Italy: from Latin Italia, from Greek Italia; of unknown origin. Perhaps an alteration of Oscan Viteliu "Italy," but meaning originally only the southwestern point of the peninsula. Traditionally said to be from Vitali, name of a tribe that settled in Calabria, whose name is perhaps somehow connected with Latin vitulus "calf." Or perhaps the country name is directly from vitulus as "land of cattle," or it might be from an Illyrian word, or an ancient or legendary ruler Italus. The modern nation dates from events of 1859-60 and was completed by the addition of Venetia in 1866 and Rome in 1870.
Issei (n.): c. 1930s, collective term used among Japanese in U.S. for first-generation immigrants, in Japanese literally "first generation," related to ichi "one."
issuance (n.): "act of issuing," 1823, American English, from issue (v.) + -ance.