Online Etymology Dictionary

J: the letter is a late modification of Roman -i-, originally a scribal creation in continental Medieval Latin to distinguish small -i- in cursive writing from the strokes of other letters, especially in the final positions of words. But in English, -y- was used for this, and -j- was introduced c.1600-1640 to take up the consonantal sound that had evolved from -i- since Late Latin times. This usage first was attested in Spanish, where it was in place before 1600. English dictionaries continued to lump together words beginning in -i- and -j- until 19c., and -j- formerly was skipped when letters were used to express serial order.

Used in modern writing to represent Latin -i- before a, e, o, u in the same syllable, which in Latin was sounded as the consonant in Modern English you, yam, etc., but the custom is controversial among Latinists:
The character J, j, which represents the letter sound in some school-books, is an invention of the seventeenth century, and is not found in MSS., nor in the best texts of the Latin authors. [Lewis]
j'accuse: French, literally "I accuse," phrase made famous by Emile Zola in a public letter attacking the irregularities of the Dreyfus trial (published Jan. 13, 1898).
jab (v.): 1825, "to thrust with a point," Scottish variant of job "to strike, pierce, thrust," from Middle English jobben "to jab, thrust, peck" (late 15c.), of unknown origin, perhaps echoic. Related: Jabbed; jabbing.
jab (n.): 1825, from jab (v.). Meaning "a punch with the fist" is from 1889. Sense of "injection with a hypodermic needle," beloved by headline writers, is from 1914.
jabber (v.): 1650s, spelling variant of Middle English jablen (c.1400), also javeren, jaberen, probably ultimately echoic. Related: Jabbered; jabbering. The noun is 1727, from the verb.
Jabberwocky: 1872, nonsense word (perhaps based on jabber) coined by Lewis Carroll, for the poem of the same name, which he published in "Through the Looking-Glass." The poem is about a fabulous beast called the Jabberwock.
jabot (n.): frill of a shirt, 1823, from French jabot "grizzard (of a bird), frill on a shirt front" (16c.), of unknown origin. Klein suggests a connection with gaver "to cram, gorge," and thus ultimately with English jaw.
jabroni (n.): c.2000, professional wrestling slang for one whose main purpose is to make the better-known wrestlers of the organization look good; he or she does this by losing to them. More commonly known as a jobber (though some enthusiasts claim there is a difference), and perhaps a mock-Italianized form of that word (but cf. jaboney "naive person, immigrant, hoodlum," of unknown origin, American English, in use c.1990).
Jobber -- A performer who regularly loses on television and doesn't receive much if any push. A comparable term for such a performer is jabroni, which is a favorite catch-phrase of Dwayne "The Rock" Johnson. To soften the blow of such labels, some wrestling promotions refer to jobbers as enhancement talent. Carpenter was the phrase used by earlier generations. ["The Professional Wrestlers' Instructional and Workout Guide," 2005]
jacaranda (n.): tropical American tree, 1753, from Portuguese jacarandá, from Tupi yacaranda.
jacinth (n.): c.1200, a blue gem (occasionally a red one), from Old French jacinte "hyacinth; jacinth" (see hyacinth). In modern use, a reddish-orange gem.
Jack: masc. proper name, 1218, probably an anglicization of Old French Jacques (which was a diminutive of Latin Jacobus; see Jacob), but in English the name always has been associated with Johan, Jan "John," and some have argued that it is a native formation.

Alliterative coupling of Jack and Jill is from 15c. (Ienken and Iulyan). In England, applied familiarly or contemptuously to anybody (especially one of the lower classes) from late 14c. Later used especially of sailors (1650s; Jack-tar is from 1781). In U.S., as a generic name addressed to an unknown stranger, attested from 1889.
jack (n.): late 14c., jakke "a mechanical device," from the masc. name Jack. The proper name was used in Middle English for "any common fellow" (mid-14c.), and thereafter extended to various appliances replacing servants (1570s). Used generically of men (jack-of-all-trades, 1610s), male animals (1620s, see jackass, jackdaw, etc.), and male personifications (1520s, e.g. Jack Frost, 1826).

As the name of a device for pulling off boots, from 1670s. The jack in a pack of playing cards (1670s) is in German Bauer "peasant." Jack shit "nothing at all" is attested by 1968, U.S. slang. The plant jack-in-the-pulpit is attested by 1837. Jack the Ripper was active in London 1888. The jack of Union Jack is a nautical term for "small flag at the bow of a ship" (1630s).
jack (v.): 1860, jack up "hoist, raise," American English, from the noun (see jack (n.)). Figurative sense "increase (prices, etc.)" is 1904, American English. Related: Jacked; jacking. Jack off (v.) "to masturbate" is attested from 1916, probably from jack (n.) in the sense of "penis."
Jack o'lantern (n.): also jack-o-lantern, 1660s, a local name for a will-o-the-wisp (Latin ignis fatuus), mainly attested in East Anglia but also in southwestern England. The extension to carved pumpkins is attested by 1834, American English.
Jack Russell: type of terrier (not recognized as a distinct breed), 1907, named for the Rev. John Russell (1795-1883) of Devonshire, "the sporting parson."
jack-in-the-box (n.): 1560s, originally a name for a sharp or cheat, "who deceived tradesmen by substituting empty boxes for others full of money" [Robert Nares, "A Glossary of Words, Phrases, Names, and Allusions," London, 1905]. As a type of toy, it is attested from 1702.
jack-knife (n.): also jackknife, large pocket knife, 1711, probably American English, "perh[aps] associated with some sense of JACK sb.1, but cf. jackleg knife" [OED]; see jack + knife (n.). Jackleg was a U.S. colloquial term of contempt from c.1850. On another theory, so called because it originally was associated with sailors. As a kind of swimming dive, from 1922. As a type of tractor-trailer accident, 1966. Both from the notion of folding, as the knife does.
jack-knife (v.): 1776, "to stab," from jack-knife (n.). Intransitive meaning "to fold or bend" the body is said to date from the time of the American Civil War. The truck accident verbal sense is from 1949. Related: Jackknifed; jackknifing.
jackal (n.): c.1600, from Turkish çakal, from Persian shaghal, from or cognate with Sanskrit srgala-s, literally "the howler." Figurative sense of "skulking henchman" is from the old belief that jackals stirred up game for lions.
jackanapes (n.): mid-15c., "a monkey," also "an impertinent, conceited fellow;" apparently from Jack of Naples, but whether this is some specific personification or folk etymology of jack (n.) + ape is unknown. See note in OED.
jackass (n.): mule ass, 1727, from jack (n.) + ass (n.1). Meaning "stupid person" is attested from 1823.
jackboot (n.): also jack-boot, 1680s, type of large, strong cavalry boot of 17c.-18c., later a type worn by German military and para-military units in the Nazi period. From jack (n.), though the exact sense here is unclear + boot (n.1). Figurative of military oppression since 1768. Related: Jackbooted.
jackdaw (n.): 1540s, the common name of the daw (Corvus monedula), "which frequents church towers, old buildings, etc.; noted for its loquacity and thievish propensities" [OED]. See jack (n.) + daw.
In modern times, parrots are almost the only birds that have the gift of speech, though connoisseurs are not ignorant that starlings and jackdaws have good abilities in that way, when properly educated. ["Chambers' Home Book and Pocket Miscellany," 1853]
jacket (n.): mid-15c., "short garment for men," from Middle French jaquet "short coat with sleeves," diminutive of Old French jaque, a kind of tunic, probably from Jacque, the male proper name, also the generic name of a French peasant (see jacquerie), but possibly associated with jaque (de mailles) "short, tight-fitting coat," originally "coat of mail," from Spanish jaco, from Arabic shakk "breastplate." Iakke "a short, close-fitting stuffed or quilted tunic, often serving as a defensive garment" is attested in English from late 14c., and by c.1400 was being used for "woman's short tunic." Meaning "paper wrapper of a book" is first attested 1894.
jackhammer (n.): also jack hammer, 1903, from jack (n.) + hammer (n.). As a verb by 1965.
jackpot (n.): also jack-pot, "big prize," 1944, from slot machine sense (1932), from obsolete poker sense (1881) of antes that begin when no player has a pair of jacks or better; from jack (n.) in the card-playing sense + pot (n.1). Earlier, in criminal slang, it meant "trouble," especially "an arrest" (1902).
The regular Draw-Poker game is usually varied by occasional Jack-Pots, which are played once in so many deals, or when all have refused to play, or when the player deals who holds the buck, a marker placed in the pool with every jack-pot. In a jack-pot each player puts up an equal stake and receives a hand. The pot must then be opened by a player holding a hand of the value of a pair of knaves (jacks) or better. If no player holds so valuable a hand the deal passes and each player adds a small sum to the pot or pool. When the pot is opened the opener does so by putting up any sum he chooses, within the limit, and his companions must pay in the same amount or "drop." They also possess the right to raise the opener. The new cards called for are then dealt and the opener starts the betting, the play proceeding as in the regular game. ["Encyclopaedia Britannica," 11th ed., 1911, "Poker." The article notes "Jack-Pots were introduced about 1870."]
jackrabbit (n.): 1863, American English, shortening of jackass-rabbit (1851), so called for its long ears. Proverbial for bursts of speed (up to 45 mph).
Jacksonian: 1824, of or in the character of U.S. politician Andrew Jackson (1767-1845). The surname is recorded from early 14c., literally "son of a man named Jack."
Jacob: masc. proper name, name of Old Testament patriarch, son of Isaac and Rebecca and father of the founders of the twelve tribes, from Late Latin Iacobus, from Greek Iakobos, from Hebrew Ya'aqobh, literally "one that takes by the heel" (Gen. xxviii:12), a derivative of 'aqebh "heel." The most popular name for boys born in the U.S. from 1999 through 2008. Jacob's ladder, in various transferred uses from 1733, is from Gen. xxviii:12.
Jacobean: 1770, literally "of James" (king or apostle), later especially "of the literary and architectural style of the time of James I" (1844). See James.
Jacobin: early 14c., of the order of Dominican friars whose order built its first convent near the church of Saint-Jacques in Paris, from Old French Jacobin (13c.) "Dominican friar," also, in the Middle East, "a Copt;" see Jacob. The Revolutionary extremists took up quarters there October 1789. Used generically of radicals and allegedly radical reformers since 1793. Related: Jacobinism.
Jacquard: 1841, from Joseph Marie Jacquard (1752-1834) of Lyons, inventor of new weaving technology c.1800.
jacquerie (n.): 1520s, from Middle French jacquerie "peasants or villeins collectively," from Jacques, the proper name, which is used as Jack is used in English, in the sense of "any common fellow." So, also, "the rising of the northern French peasants against the nobles, 1357-8," from a French usage. Etymologically, Jacques is from Late Latin Iacobus (see Jacob).
Jacuzzi (n.): 1966, U.S. proprietary name, from Jacuzzi Bros., Inc., Little Rock, Arkansas.
jade (n.1): ornamental stone, 1721, earlier iada (1590s), from French le jade, error for earlier l'ejade, from Spanish piedra de (la) ijada (1560s), "stone of colic, pain in the side" (jade was thought to cure this), from Vulgar Latin *iliata, from Latin ilia (plural) "flanks, kidney area" (see ileum).
jade (n.2): "worn-out horse," late 14c., "cart horse," of uncertain origin. Barnhart suggests a variant of yaid, yald "whore," literally "mare," from a Scandinavian source akin to Old Norse jalda "mare," from Finno-Ugric (cf. Mordvin al'd'a "mare"). But OED finds the assumption of a Scandinavian connection "without reason." As a term of abuse for a woman, it dates from 1550s.
jade (v.): "to weary, tire out, make dull," c.1600, from jade (n.2). Related: Jaded; jading.
jaded (adj.): "bored by continual indulgence," 1630s; past participle adjective from jade (v.).
jag (n.1): "period of unrestrained activity," 1887, American English, perhaps via intermediate sense of "as much drink as a man can hold" (1670s), from earlier meaning "load of hay or wood" (1590s), of unknown origin. Used in U.S. colloquial speech from 1834 to mean "a quantity, a lot."
jag (n.2): "slash or rend in a garment," c.1400, of unknown origin.
jager (n.): "German sharpshooter," 1776, from German jäger, literally "huntsman," from jagen "to hunt," from Old High German jagon, related to Old Frisian jagia, Dutch jagen "to hunt," Old Norse jaga "to drive, to move to and fro" (see yacht). Applied to riflemen and sharpshooters in the German and Austrian armies.
jagged (adj.): mid-15c., from verb jaggen (c.1400) "to pierce, slash, cut; to notch or nick; cut or tear unevenly," Scottish and northern English, of unknown origin. Originally of garments with regular "toothed" edges; meaning "with the edge irregularly cut" is from 1570s. Related: Jaggedly; jaggedness.
jaguar (n.): big cat of the Americas (Felis onca), c.1600, from Portuguese jaguar, from Tupi jaguara, said to be a name "denoting any larger beast of prey" [Klein]. Also a type of British-made car; in this sense the abbreviation Jag is attested from 1959.
Jah (n.): 1530s, a form of Hebrew Yah, short for Yahweh "Jehovah." Used in some English bibles. Cognate with the second element in hallelujah and in Elijah.
jai alai (n.): 1902, American English, originally in a Cuban context, from Basque, from jai "celebration" + alai "merry."
jail (n.): late 13c., gayhol, from Old North French gaiole and Old French jaole, both meaning "a cage, prison," from Medieval Latin gabiola, from Late Latin caveola, diminutive of Latin cavea "cage, enclosure, stall, coop" (see cave (n.)). Both forms carried into Middle English; now pronounced "jail" however it is spelled. Persistence of Norman-derived gaol (preferred in Britain) is "chiefly due to statutory and official tradition" [OED].
jail (v.): "to put in jail," c.1600, from jail (n.). Related: Jailed; jailing.
jailbait (n.): also jail bait, jail-bait, "girl under the legal age of consent," 1930, from jail (n.) + bait (n.).
jailbird (n.): 1610s, based on an image of a caged bird; from jail (n.) + bird (n.1).
jailbreak (n.): also jail-break, "prison escape," 1872, perhaps 1828, from jail (n.) + break (n.).