I (pron.)
12c., a shortening of Old English ic, the first person singular nominative pronoun, from Proto-Germanic *ek (source also of Old Frisian ik, Old Norse ek, Norwegian eg, Danish jeg, Old High German ih, German ich, Gothic ik), from PIE *eg- "I," nominative form of the first person singular pronoun (source also of Sanskrit aham, Hittite uk, Latin ego (source of French Je), Greek ego, Russian ja, Lithuanian ).

Reduced to i by mid-12c. in northern England, later everywhere; the form ich or ik, especially before vowels, lingered in northern England until c. 1400 and survived in southern dialects until 18c. It began to be capitalized mid-13c. to mark it as a distinct word and avoid misreading in handwritten manuscripts.
The reason for writing I is ... the orthographic habit in the middle ages of using a 'long i' (that is, j or I) whenever the letter was isolated or formed the last letter of a group; the numeral 'one' was written j or I (and three iij, etc.), just as much as the pronoun. [Otto Jespersen, "Growth and Structure of the English Language," p.233]
The dot on the "small" letter -i- began to appear in 11c. Latin manuscripts to distinguish the letter from the stroke of another letter (such as -m- or -n-). Originally a diacritic, it was reduced to a dot with the introduction of Roman type fonts. The letter -y- also was written with a top dot in Old English and early Middle English, during the centuries when -i- tended to be written with a closed loop at the top and thus was almost indistinguishable from the lower-case thorn (þ). In names of U.S. highways (by 1966) it is short for Interstate (adj.).
I Ching
1876, from Chinese, said to mean "Book of Changes."
i'nt
also i'n't, 18c., contraction representing a casual pronunciation of isn't it.
I'se
1847 in representations of African-American vernacular, a contraction of I is (see is), irregular for I am. In Scottish and northern English, a colloquial or dialectal contraction of I shall (1796).
I've
contraction of I have, 1742, first attested in Richardson's "Pamela."
I-beam (n.)
1869; see beam (n.). So called for its shape. I-bar is from 1890; also I-rail (1873).
I.D.
also ID (but pronounced as separate letters), short for identification, attested from 1955.
i.e.
abbreviation of Latin id est, literally "that is;" used in English in the sense of "that is to say." Latin id "it" is from PIE pronominal stem *i- (see yon). For est, see is.
I.H.S.
Old English, from Medieval Latin, representing Greek abbreviation of IHSOUS "Jesus," in which the character -H- is the capital of the Greek vowel eta. The Roman form would be I.E.S. Mistaken for a Latin contraction in the Middle Ages, after its Greek origin was forgotten, and sometimes treated as short for Iesus Hominum Salvator "Jesus Savior of Men." Alternative version I.H.C. (terminal -s- often was indicated in later Greek with a character resembling -c-) is found on vestments from 950 C.E., and may be the source of the H. in slang Jesus H. Christ.
i.n.r.i.
ecclesiastical inscription, it stands for Iesus Nazarenus, Rex Iudaeorum ("Jesus of Nazareth, King of the Jews," John xix.19).
I.Q.
1922, abbreviation of intelligence quotient, a 1921 translation of German Intelligenz-quotient, coined 1912 by German psychologist William L. Stern (1871-1938).
Intelligence is a general capacity of an individual consciously to adjust his thinking to new requirements: it is general mental adaptability to new problems and conditions of life. [Stern, "The Psychological Methods of Testing Intelligence," 1914]
Earlier, i.q. was an abbreviation of Latin idem quod "the same as."
I.R.A. (2)
also IRA, initialism (acronym) for individual retirement account, attested from 1974.
I.R.A. (1)
also IRA, 1921, initialism (acronym) for Irish Republican Army, the full name of which attested from 1919.
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.
iambic
1570s (n.); 1580s (adj.), from Late Latin iambicus, from Greek iambikos, from iambos "metrical foot of one unaccented followed by one accented syllable; an iambic verse or poem," from iaptein "to assail, attack" (in words), literally "to put forth, send forth" (in reference to missiles, etc.). The meter of invective and lampoon in classical Greek since it was first used 7c. B.C.E. by Archilochus, whose tomb, Gaetulicus says, is haunted by wasps.
Ian
masc. proper name, Scottish form of John (q.v.).
Iapetus
a titan, son of Uranus and Gaia.
iatro-
word-forming element meaning "a physician; medicine; healing," from Greek iatros "healer, physician" (see -iatric).
iatrogenic (adj.)
"induced by a physician," 1920, from iatro- + -genic.
Iberia
from Latin Iberia, the ancient name of the large southwestern peninsula of Europe, from Greek Iberes, the name of a Celtic people of ancient Spain. An identical name was given to an Asiatic people near the Caucasus in what is now Georgia. Of unknown origin in both uses, but the word as applied in Spain is believed to be related to that of the River Ebro. Related: Iberian (c. 1600 as a noun; 1610s as an adjective).
ibex (n.)
"chamois, wild goat of the Alps and Apennines," c. 1600, from Latin ibex, which probably is from a pre-Latin Alpine language. The German Steinbock.
ibid. (adv.)
"at the place or in the book already mentioned" (used to avoid repetition of references), 1660s, abbreviation of Latin ibidem "in the same place, just there," from ibi "there," pronominal adverb of place, + demonstrative suffix -dem. Also ibid, but properly with the period.
ibis (n.)
stork-like bird, late 14c., from Latin ibis (plural ibes), from Greek ibis, from Egyptian hab, a sacred bird of Egypt.
IBM
also (in early use) I.B.M., initialism (acronym) attested by 1921 from International Business Machines Co.; the company name in use from 1918.
ibogaine (n.)
nerve stimulant, 1901, from French ibogaine, from iboga, Congolese name of the shrub from which the chemical is extracted, + chemical suffix -ine (2).
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.
ICBM (n.)
also I.C.B.M., 1955, initialism (acronym) for Inter-Continental Ballistic Missile. IBM in the same sense is from 1954.
ice (n.)
Old English is "ice, piece of ice" (also the name of the Anglo-Saxon rune for -i-), from Proto-Germanic *is- "ice" (source also of Old Norse iss, Old Frisian is, Dutch ijs, German Eis), of uncertain origin; possible relatives are Avestan aexa- "frost, ice," isu- "frosty, icy;" Afghan asai "frost." Slang meaning "diamonds" is attested from 1906.

Modern spelling begins to appear 15c. and makes the word look French. On ice "kept out of the way until wanted" is from 1890. Thin ice in the figurative sense is from 1884. To break the ice "to make the first opening to any attempt" is from 1580s, metaphoric of making passages for boats by breaking up river ice though in modern use it usually has implications of "cold reserve." Ice-fishing is from 1869.
ice (v.)
c. 1400, ysen, "cover with ice," from ice (n.). Related: Iced; icing.
ice age (n.)
1855, from ice (n.) + age (n.). Perhaps translating German Eiszeit (1837). An earlier term in the same sense was glacial epoch (1841). Local scientific men had noticed from the late 18c. evidence that the Alpine glaciers once had been much larger; in the 1830s stray boulders, moraines, and polished bedrock in northern Europe (formerly interpreted as relics of catastrophic floods) began to be understood as revealing the former presence of a large ice cap there. When Agassiz, a convert to the theory, came to America in 1846 he found similar evidence in New England. The glacial theory and the notion that there had been several worldwide ice ages seems to have been generally accepted by the 1870s.
ice-bound (adj.)
1650s, from ice (n.) + bound (adj.1).
ice-box (n.)
also icebox, 1839, "an ice chest," later "the small compartment in a refrigerator containing the ice," from ice (n.) + box (n.).
ice-cap (n.)
1859 in geology, from ice (n.) + an extended sense of cap (n.).
Ice-Capade (n.)
1941, originally a film title, from ice (n.) + a punning play on escapade.
ice-chest (n.)
1839, originally a wooden chest lined with zinc, from ice (n.) + chest (n.).
ice-cold (adj.)
Old English isceald; see ice (n.) + cold (adj.).
ice-cream (n.)
1744, earlier iced cream (1680s), from ice (n.) + cream (n.).
ice-cube (n.)
1904, from ice (n.) + cube (n.).
ice-house (n.)
1680s, from ice (n.) + house (n.).
ice-pick (n.)
1858, from ice (n.) + pick (n.1).
ice-skate (v.)
1690s, from ice (n.) + skate (n.2). The verb usually was simply skate until the advent of roller-skating mid-18c. made distinction necessary. A run of severe winters that froze over the Thames in the late 17c. made ice-skating popular in England. Related: ice-skates (1862).
ice-water (n.)
1722, from ice (n.) + water (n.1).
iceberg (n.)
1774, "glacier humped like a hill;" 1820 as "detached piece of a glacier or ice pack at sea," partial loan-translation of Dutch ijsberg, literally "ice mountain," from ijs "ice" (see ice (n.)) + berg "mountain" (from PIE root *bhergh- (2) "high," with derivatives referring to hills and hill-forts.). Similar formation in Norwegian isberg, Danish isbjerg.

Earlier English terms were sea-hill (1690s), island of ice (1610s). Phrase tip of the iceberg in a figurative sense (in allusion to most of it being unseen underwater) first recorded 1962. Iceberg lettuce attested from 1893, apparently originally a trade name.
Iceland
c. 1200, so called for its ice-choked fjords. Related: Icelander; Icelandic.
iceman (n.)
"dealer in ice," 1844, from ice (n.) + man (n.).
ichneumon (n.)
1570s, "weasel-like animal of Egypt," from Latin ichneumon, from Greek ikhneumon "ichneumon," literally "searcher, tracker," perhaps so called because it hunts crocodile eggs, from ikhneuein "hunt for, track," from ikhnos "a track, footstep, trace, clue," of unknown origin. Used by Aristotle for a species of wasp that hunts spiders (a sense attested in English from 1650s).
ichnolite (n.)
"stone presenting a fossil footprint," 1841, from Latinized form of Greek ikhnos "a track, footprint" + -lite. Ichnite in the same sense is from 1854. Ichnology, "scientific study of fossil footprints," is from 1837.
So numerous have been the discoveries of fossil footmarks in Europe within a few years past, and so many species occur in this country, that it will be at least convenient to have them designated by some appropriate scientific terms, and to arrange them in systematic order. I propose the term Ichnolite ... to include them all and to be the name of the Class. [Edward Hitchcock, LL.D., "Final Report on the Geology of Massachusetts," 1841]
ichnomancy (n.)
"the reading of traces of footsteps to determine the nature and peculiarities of what made them," 1855, from Latinized form of Greek ikhnos "a track, footprint" + -mancy.
ichor (n.)
"ethereal fluid that serves for blood in the veins of the gods," 1630s, from French ichor (16c.) or Modern Latin ichor, from Greek ikhor, of unknown origin, possibly from a non-Indo-European language. Related: Ichorous.
ichthyo-
word-forming element meaning "fish," from Latinized form of Greek ikhthys "a fish" (in plural, "a fish-market"), from PIE root *dhghu- "fish" (source also of Armenian jukn, Lithuanian žuvis).