Online Etymology Dictionary

irritability (n.): 1755, from irritable + -ity.
irritable (adj.): 1660s, from French irritable and directly from Latin irritabilis "easily excited," from irritare (see irritate). Related: Irritably.
irritant (adj.): 1630s, from Latin irritantem (nominative irritans), present participle of irritare (see irritate). As a noun, from 1802.
irritate (v.): 1530s, "stimulate to action, rouse, incite," from Latin irritatus, past participle of irritare "excite, provoke." An earlier verb form was irrite (mid-15c.), from Old French irriter. Meaning "annoy, make impatient" is from 1590s. Related: Irritated; irritating.
irritation (n.): early 15c., in reference to sores and morbid swelling, from Middle French irritation or directly from Latin irritationem (nominative irritatio) "incitement, irritation," noun of action from past participle stem of irritare (see irritate).
irrupt (v.): "to break into," 1855, back-formation from irruption or else from Latin irruptus, past participle of irrumpere (see irruption).
irruption (n.): 1570s, from Middle French irruption or directly from Latin irruptionem (nominative irruptio) "a breaking in, bursting in, invasion," noun of action from past participle stem of irrumpere, from assimilated form of in- "into, in, on, upon" (see in- (2)) + rumpere (see rupture). Frequently confused with eruption.
IRS: also I.R.S., initialism 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 of be, Old English is, from Germanic stem *es- (cf. Old High German, German, Gothic ist, Old Norse es, er), from PIE *es-ti- (cf. Sanskrit asti, Greek esti, Latin est, Lithuanian esti, Old Church Slavonic jesti), from PIE root *es- "to be." Old English lost the final -t-. See be. Until 1500s, pronounced to rhyme with kiss.
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 name in 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") from 1600; the Isabella who gave her name to it has not been identified. Related: Isabelline.
isagoge (n.): 1650s, from Latin isagoge, from Greek eisagoge "introduction (into court), importation (of goods)," from eis "into" + agoge "a leading," from agein "to lead" (see act). Related: Isagogic; isagogical (1520s).
Isaiah: masc. proper name, name of a biblical prophet, from Hebrew Yesha'yah, abbreviated form of Yesha'yahu, literally "salvation of the Lord," from yesha, yeshua "salvation, deliverance."
ISBN: 1969, acronym for International Standard Book Number.
Iscariot: "traitor," 1640s, from the 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).
ischaemia (n.): also ischemia, 1866 (but as far back as 1660s in form ischaimes), from medical Latin ischaemia, from ischaemus "stopping blood," from Greek iskhaimos "stanching or stopping of blood," from iskhein "to hold" + 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," of unknown origin.
ish kabibble: 1913, "I should worry," 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: name for the 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, 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.
Ishtar: ancient Sumero-Babylonian goddess of love and fertility, counterpart of Phoenician Astarte (q.v.), from Akkad. Ishtar.
Isidore: masc. proper name, from French, from Latin Isidorus, from Greek Isidoros, literally "gift of Isis," from 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.
isinglass (n.): 1520s, said to be perversion of Dutch huysenblas, literally "sturgeon bladder," from huysen "sturgeon" + blas "bladder;" so called because the substance was obtained from it.
Islam: "religious system revealed by Muhammad," 1818, 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), which in part is from Ishmaelite, a name formerly given (especially by Jews) to Arabs, as descendants of Ishmael (q.v.), and in part from Arabic Ismailiy, name of the Shiite sect that after 765 C.E. followed the Imamship through descendants of Ismail (Arabic for Ishmael), eldest son of Jafar, the sixth Imam. The Ismailians were not numerous, but among them were the powerful Fatimid dynasty in Egypt and the Assassins, both of whom loomed large in European imagination.
Islamic (adj.): 1791, from Islam + -ic.
island (n.): 1590s, earlier yland (c.1300), from Old English igland "island," from ieg "island" (from Proto-Germanic *aujo "thing on the water," from PIE *akwa- "water;" see aqua-) + land "land." Spelling modified 15c. by association with similar but unrelated isle. An Old English cognate was ealand "river-land, watered place, meadow by a river." 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]. Related: Islander.
isle (n.): late 13c., from Old French ile, earlier isle, from Latin insula "island," of uncertain origin, perhaps (as the Ancients guessed) from in salo "(that which is) in the sea," from ablative of salum "the open sea." 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).
iso-: word-forming element meaning "equal, similar, identical; isometric," from comb. form of Greek isos "equal to, the same as" (e.g. isometor "like one's mother"). 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 barys "heavy" (see grave (adj.)). Line connecting places with the same barometric pressure at the same time.
isocephalic (adj.): "having the heads of the principal figures at about the same level," from Greek isokephalos "like-headed," from isos "equal" (see iso-) + kephale "head" (see cephalo-).
isochronous (adj.): 1706, with suffix -ous, from Modern Latin isochronus, from Greek isokhronos "equal in 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 power," from isokrates "of equal power, possessing equal rights," from isos "equal" (see iso-) + -kratia "power, rule, authority" (see -cracy). Related: Isocratic.
isokinetic (adj.): 1958, from iso- + kinetic.
isolate (v.): by 1786, a new 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 from 1890, from earlier adjectival use (1819).
isolated (adj.): 1763, from French isolé "isolated" (17c.) + English -ated (see -ate (2)). The French word is from Italian isolato, from Latin insulatus "made into an island," from insula "island." The French word was used at first in English (isole, also isole'd, c.1750), then after isolate became an English word, isolated became its past participle.
isolation (n.): 1800, noun of action from isolate, or else from French isolation, noun of action from isoler (see isolated).
isolationist: in reference to U.S. foreign policy, 1899 (earlier in reference to treatment of leprosy), from isolation + -ist. Isolationism is attested by 1922.
Isold: fem. proper name, French Isolt, Iseut, of Germanic origin, literally "ice-rule," from *is "ice" + *waltan (see wield).
isomer (n.): 1866, back-formation from isomeric; cf. Greek isomeres "sharing equality," from iso- (see iso-) + meros "part, share" (see merit (n.)).
isometric (adj.): 1838, literally "of the same measure," coined from Greek isos "equal" (see iso-) + metron "measure" (see meter (n.2)). Originally a method of using perspective in drawing; the physiological sense relating to muscular action is from 1891, from German isometrisch in this sense (1882).
isometrics (n.): 1962, American English, from isometric; also see -ics.
isometry (n.): 1941, from Greek isometria "equality of measure," from iso- (see iso-) + metria "a measuring" (see -metry).
isomorphic (adj.): 1862, from iso- + Greek morphe (see Morpheus).
isomorphism (n.): from German Isomorphismus, 1828, coined by German chemist Eilhard Mitscherlich (1794-1863) from isomorph; see isomorphic. Related: Isomorph.
isopod (n.): 1835, from French isopode, from Latin isopoda (neuter plural), from Greek iso- (see iso-) + pod-, stem of pous "foot" (see foot (n.)).
isosceles (adj.): "having two equal sides," 1550s, from Late Latin isosceles, from Greek isoskeles "with equal sides," from isos "equal" (see iso-) + skelos "leg" (see scalene).
isostasy (n.): 1889, from iso- + Greek stasis "setting, weighing, standing" (see stet). Related: Isostatic.
isotherm (n.): 1860, from French isotherme (Humboldt, 1817), from Greek iso- (see iso-) + therme "heat" (see thermal).