J Look up J at Dictionary.com
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 into English 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]
In words from Hebrew, -j- represents yodh, which was equivalent to English consonantal y (as in hallelujah) but in many names it later was conformed in sound to the -j- of Latin or French (Jesus).
j'accuse Look up j'accuse at Dictionary.com
jab (v.) Look up jab at Dictionary.com
jab (n.) Look up jab at Dictionary.com
jabber (v.) Look up jabber at Dictionary.com
Jabberwocky Look up Jabberwocky at Dictionary.com
jabot (n.) Look up jabot at Dictionary.com
frill of a shirt, 1823, from French jabot "gizzard (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 (n.).
jabroni (n.) Look up jabroni at Dictionary.com
jacaranda (n.) Look up jacaranda at Dictionary.com
tropical American tree, 1753, from Portuguese jacarandá, from Tupi yacaranda.
jacinth (n.) Look up jacinth at Dictionary.com
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 Look up Jack at Dictionary.com
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.) Look up jack at Dictionary.com
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, such as 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.) Look up jack at Dictionary.com
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.) Look up Jack o'lantern at Dictionary.com
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 Look up Jack Russell at Dictionary.com
jack-in-the-box (n.) Look up jack-in-the-box at Dictionary.com
jack-knife (n.) Look up jack-knife at Dictionary.com
also jackknife, large pocket knife, 1711, probably American English, "perh[aps] associated with some sense of JACK sb.1, but compare 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.) Look up jack-knife at Dictionary.com
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.) Look up jackal at Dictionary.com
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.) Look up jackanapes at Dictionary.com
jackass (n.) Look up jackass at Dictionary.com
mule ass, 1727, from jack (n.) + ass (n.1). Meaning "stupid person" is attested from 1823.
jackboot (n.) Look up jackboot at Dictionary.com
jackdaw (n.) Look up jackdaw at Dictionary.com
jacket (n.) Look up jacket at Dictionary.com
jackhammer (n.) Look up jackhammer at Dictionary.com
also jack hammer, 1903, from jack (n.) + hammer (n.). As a verb by 1965.
jackpot (n.) Look up jackpot at Dictionary.com
jackrabbit (n.) Look up jackrabbit at Dictionary.com
1863, American English, shortening of jackass-rabbit (1851), so called for its long ears. Proverbial for bursts of speed (up to 45 mph).
Jacksonian Look up Jacksonian at Dictionary.com
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 Look up Jacob at Dictionary.com
Jacobean Look up Jacobean at Dictionary.com
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 Look up Jacobin at Dictionary.com
early 14c., in reference to an order of Dominican friars, from Old French Jacobin (13c.) "Dominican friar" (also, in the Middle East, "a Copt"); so called because the order built its first convent near the church of Saint-Jacques in Paris (the masc. proper name Jacques is from Late Latin Iacobus; see Jacob). The Revolutionary extremists made their club headquarters there October 1789 and supported Robespierre during the Terror. It was suppressed in November 1794. In English, used generically of radicals and allegedly radical reformers since 1793. Related: Jacobinism.
Jacquard Look up Jacquard at Dictionary.com
1841, from Joseph Marie Jacquard (1752-1834) of Lyons, inventor of new weaving technology c. 1800.
jacquerie (n.) Look up jacquerie at Dictionary.com
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.) Look up Jacuzzi at Dictionary.com
jade (n.1) Look up jade at Dictionary.com
jade (n.2) Look up jade at Dictionary.com
jade (v.) Look up jade at Dictionary.com
jaded (adj.) Look up jaded at Dictionary.com
jag (n.1) Look up jag at Dictionary.com
"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) Look up jag at Dictionary.com
"slash or rend in a garment," c. 1400, of unknown origin.
jager (n.) Look up jager at Dictionary.com
"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. Englished as yager, yaeger from 1804.
jagged (adj.) Look up jagged at Dictionary.com
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.) Look up jaguar at Dictionary.com
Jah (n.) Look up Jah at Dictionary.com
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.) Look up jai alai at Dictionary.com
jail (n.) Look up jail at Dictionary.com
jail (v.) Look up jail at Dictionary.com
jailbait (n.) Look up jailbait at Dictionary.com
also jail bait, jail-bait, "girl under the legal age of consent," 1930, from jail (n.) + bait (n.).
jailbird (n.) Look up jailbird at Dictionary.com
jailbreak (n.) Look up jailbreak at Dictionary.com
also jail-break, "prison escape," 1872, perhaps 1828, from jail (n.) + break (n.).