Online Etymology Dictionary

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.
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.
irritate (v.): 1530s, "stimulate to action, rouse, incite," from Latin irritatus, past participle of irritare "excite, provoke, annoy;" according to de Vaan, probably a verb from Proto-Italic *rito- "stirred," from the same PIE root that produced English run (v.). Meaning "annoy, make impatient" in English is from 1590s. The earlier verb in English was irrite (mid-15c.), from Old French irriter. Related: Irritated; irritating.
irritating (adj.): "that causes annoyance," 1707, present participle adjective from irritate (v.). Related: Irritatingly. Earlier adjective forms were irritative (1680s), irritatory (1650s).
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.
irrumation (n.): "a putting of the erect penis in the mouth of another," 1866, from past participle stem of Latin irrumare, literally "to give to suck" (originally of the breast), from in- "in" + ruma "teat, female breast." Related: Irrumate (v.); irrumator.
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."
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.
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.
Irwin: masc. proper name, Old English Eoforwine "boar-friend;" often confused with Irving, Irvin, which are from Irvine, Ayrshire, or Irving, Dumfries.
is (v.): third person singular present indicative of be, Old English is, from Germanic stem *es- (source also of Old High German, German, Gothic ist, Old Norse es, er), from PIE *es-ti- (source also of Sanskrit asti, Greek esti, Latin est, Lithuanian esti, Old Church Slavonic jesti), from PIE root *es- "to be." Old English lost the final -t-.

Until 1500s, pronounced to rhyme with kiss. Dialectal use for all persons (I is) is in Chaucer. Phrase it is what it is, indicating resigned acceptance of an unpleasant but inevitable situation or circumstance about which nothing truly positive can be said, is attested by 2001.
Isaac: masc. proper name, name of a biblical patriarch, from Late Latin, from Greek Isaak, from Hebrew Yitzhaq, literally "he laughs," imperf. of tzahaq "he laughed."
Isabel: fem. proper name, a form of Elizabeth that seems to have developed in Provence. A popular English name in the Middle Ages; pet forms included Ibb, Libbe, Nibb, Tibb, Bibby, and Ellice. The Spanish form was Isabella, which is attested as a color name ("greyish-yellow") in English from c. 1600; the Isabella who gave her name to it has not been identified, and the usual stories are too late for the date. Related: Isabelline (adj.).
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.
Isaiah: masc. proper name, name of a biblical prophet and of the book credited to him, from Hebrew Yesha'yah, abbreviated form of Yesha'yahu, literally "salvation of the Lord," from yesha, yeshua "salvation, deliverance." Related: Isaian
ISBN: 1969, acronym for International Standard Book Number.
Iscariot: surname of Judas, betrayer of Jesus, in New Testament, from Latin Iscariota, from Greek Iskariotes, said to be from Hebrew ishq'riyoth "man of Kerioth" (a place in Palestine). In English from 1640s as a noun meaning "traitor." Related: Iscariotical.
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.
ischium (n.): "the seat bone," 1640s, from Latin, from Greek iskhion "hip joint," in plural, "the hips," probably from iskhi "loin," a word of unknown origin. Related: Ischiatic.
Isegrim: name of the wolf in Reynard and other beast-fables, from isen "iron" (see iron (n.)) + grima "mask, hood, helmet" (see grimace (n.)). In German, Isegrimm, Isengrimm.
ish kabibble: slang phrase meaning, more or less, "I don't care, I don't worry," 1913, of unknown origin, but perhaps derived from Yiddish nisht gefidlt. Said to have been popularized by comedienne Fanny Brice (1891-1951), but earliest references do not mention her.
Chicken pox doesn't poison the wellsprings of one's existence like 'Ish kabibble,' and 'I should worry.!' Do you think it's any fun to bring up children to speak decent English, and then have their conversation strewed with phrases like that and with ain'ts? Do you think I like to hear Robert talking about his little friends as 'de guys' and 'de ginks?' [Mary Heaton Vorse, "Their Little Friends," in "Woman's Home Companion," February 1916]
Ishihara: popular type of colorblindness test, 1924, from Japanese ophthalmologist Shinobu Ishihara (1879-1963), who devised it in 1917.
Ishmael: masc. proper name, biblical son of Abraham and Hagar, driven into the wilderness with his mother, from Hebrew Yishma'el, literally "God hears," from yishma, imperf. of shama "he heard." The Arabs claim descent from him. Figurative sense of "an outcast," "whose hand is against every man, and every man's hand against him" is from Gen. xvi:12. Related: Ishmaelite.
Ishtar: ancient Sumero-Babylonian goddess of love and fertility, counterpart of Phoenician Astarte (q.v.), from Akkadian Ishtar.
Isidore: masc. proper name, from French, from Latin Isidorus, from Greek Isidoros, literally "gift of Isis," from Isis (see Isis) + doron "gift" (see date (n.1)). St. Isidore, archbishop of Seville (600-636) wrote important historical, etymological, and ecclesiastical works and in 2001 was named patron saint of computers, computer users, and the Internet. Related: Isidorian.
isinglass (n.): purest commercial form of gelatin, 1520s, apparently a perversion of Dutch huysenblas, literally "sturgeon bladder," from huysen "sturgeon" + blas "bladder," from PRoto-Germanic *bles-, extended form of root *bhle- (2) "to blow, swell" (see bole). So called because the substance was obtained from the air-bladders of certain freshwater fishes.
Isis: Egyptian goddess, from Greek Isis, from Egyptian Hes, female deity identified by the Greeks with Io. She is distinguished in visual representations by the solar disc and cow horns on her head.
Islam (n.): "religious system revealed by Muhammad," 1816, from Arabic islam, literally "submission" (to the will of God), from root of aslama "he resigned, he surrendered, he submitted," causative conjunction of salima "he was safe," and related to salam "peace."
... Islam is the only major religion, along with Buddhism (if we consider the name of the religion to come from Budd, the Divine Intellect, and not the Buddha), whose name is not related to a person or ethnic group, but to the central idea of the religion. ["The Heart of Islam: Enduring Values for Humanity," Seyyed Hossein Nasr, 2002]
Earlier English names for the faith include Mahometry (late 15c.), Muhammadism (1610s), Islamism (1747), and Ismaelism (c. 1600; see Ismailite). The Ismailites were not numerous in Islam, but among them were the powerful Fatimid dynasty in Egypt and the Assassins, both of which loomed large in European imagination. This use also is in part from Ishmaelite, a name formerly given (especially by Jews) to Arabs, as descendants of Ishmael (q.v.).
Islamic (adj.): "pertaining to Islam," 1791, from Islam + -ic.
Islamist (n.): 1850, "a Muslim," from Islam + -ist. Later also "scholar of Islamic studies." By 1962 specifically as "strict fundamentalist Sunni Muslim." Islamism is attested from 1747 as "the religion of the Muslims, Islam." Islamite "a Muslim" is from 1786 (1768 as an adjective); Islamize/Islamise (v.) is from 1849.
Islamophobia (n.): "hostility or discrimination against Muslims," supposedly rooted in dread or hatred of Islam, by 1996, from Islam + -phobia, as used in Jodeaophobia, Francophobia, etc.
The term [a report by the liberal think-tank Runnymede Trust] uses, 'Islamophobia,' is so recently coined that it has yet to be recognised in the Oxford English Dictionary, but according to the trust the phenomenon it refers to 'has existed in western countries and cultures for centuries.' ["Islamophobia," Third Way, April 1997]
Related: Islamophobic; Islamophobe.
island (n.): 1590s, earlier yland (c. 1300), from Old English igland, iegland "an island," from ieg "island" (from Proto-Germanic *aujo "thing on the water," from PIE *akwa- "water;" see aqua-) + land (n.). As an adjective from 1620s.

Spelling modified 15c.-16c. by association with similar but unrelated isle. Similar formation in Old Frisian eiland, Middle Dutch eyland, German Eiland, Danish öland, etc. In place names, Old English ieg is often used of "slightly raised dry ground offering settlement sites in areas surrounded by marsh or subject to flooding" [Cambridge Dictionary of English Place-Names]. Island universe "solar system" (1846) translates German Weltinsel (von Humboldt, 1845). An Old English cognate was ealand "river-land, watered place, meadow by a river." Related: Islander.
islander (n.): "native or inhabitant of an island," 1540s, from island (n.) + -er (1).
isle (n.): late 13c., ile, from Old French ile, earlier isle, from Latin insula "island," a word of uncertain origin.

Perhaps (as the Ancients guessed) from in salo "(that which is) in the (salty) sea," from ablative of salum "the open sea," related to sal "salt" (see salt (n.)). De Vaan finds this "theoretically possible as far as the phonetics go, but being 'in the sea' is not a very precise description of what an island is; furthermore, the Indo-Europeans seem to have indicated with 'island' mainly 'river islands.' ... Since no other etymology is obvious, it may well be a loanword from an unknown language." He proposes the same lost word as the source of Old Irish inis, Welsh ynys "island" and Greek nesos "island." The -s- was restored first in French, then in English in the late 1500s.
islet (n.): 1530s, from Middle French islette (Modern French îlette), diminutive of isle (see isle).
ism (n.): "distinctive doctrine, theory, or practice," 1670s, the suffix -ism used as an independent word, chiefly disparagingly. Related: Ismatical. By the same path, ist is from 1811.
Ismailite (n.): also Ismaelite, 1570s, in reference to a Shi'ite Muslim sect, from Arabic Isma'iliy, the name of the sect that after 765 C.E. followed the Imamship through descendants of Ismail (Arabic for Ishmael), deceased eldest son of Jafar, the sixth Imam, rather than his surviving younger son.
isness (n.): "essence," 1865, in a translation of Hegel, from is + -ness.
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-).
isobar (n.): 1864, coined from Greek isos "equal" (see iso-) + baros "weight," from PIE root *gwere- (2) "heavy" (see grave (adj.)). Line connecting places with the same barometric pressure at the same time. Related: Isobaric.
isocephalic (adj.): "having the heads of the principal figures at about the same level," 1893, from Greek isokephalos "like-headed," from isos "equal" (see iso-) + kephale "head" (see cephalo-). The rule or principle of ancient Greek art that makes figures standing and on horseback have heads carved on nearly the same level.
isochromatic (adj.): "having the same color," 1817, from iso- "equal, the same" + stem of chroma + -ic.
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).
isocracy (n.): "equal power," 1650s, from Greek isokratia "equality of political rights," from isokrates "of equal power, possessing equal rights with (others)," from isos "equal" (see iso-) + -kratia "power, rule, authority" (see -cracy). Related: Isocratic.
isodynamic (adj.): "having equal power or force," 1827, from iso- "the same, equal" + dynamic (adj.).
isogenous (adj.): "having the same or similar origin," 1856; see iso- "the same, equal" + -genous.
isogloss (n.): 1925, from German Isogloss (1892); see iso- + gloss (n.2).
isokinetic (adj.): 1942, from iso- + kinetic.
isolable (adj.): 1832, from isolate (v.) + -able on model of violate/violable, etc. Isolatable is recorded from 1870.
isolate (v.): "to set or place apart, to detach so as to make alone," by 1786, a back-formation from isolated (q.v.).
The translation of this work is well performed, excepting that fault from which few translations are wholly exempt, and which is daily tending to corrupt our language, the adoption of French expressions. We have here evasion for escape, twice or more times repeated; brigands very frequently; we have the unnecessary and foolish word isolate; and, if we mistake not, paralize, which at least has crept in through a similar channel. Translators cannot be too careful on this point, as it is a temptation to which they are constantly exposed. ["The British Critic," April 1799]
As a noun, "something isolated," 1890; from earlier adjectival use (1819), which is from Italian isolato or Medieval Latin insulatus.