Etymology
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J 

10th letter of the English alphabet, pronounced "jay," as in "kay" for -k-, but formerly written out as jy, rhyming with -i- and corresponding to French ji.

One of the most stable English letters (it has almost always the same sound), it is a latecomer to the alphabet and originally had no sound value. The letter itself began as a scribal modification of Roman -i- in continental Medieval Latin. The scribes added a "hook" to small -i-, especially in the final position in a word or roman numeral, to distinguish it from the strokes of other letters. The dot on the -i- (and thus the -j-) and the capitalization of the pronoun I are other solutions to the same problems.

In English, -j- was used as a roman numeral throughout Middle English, but the letter -y- was used to spell words ending an "i" sound, so -j- was not needed to represent a sound. Instead, it was introduced into English c. 1600-1640 to take up the consonantal sound that had evolved from the Roman i- since Late Latin times. In Italian, g- was used to represent this, but in other languages j- took the job. This usage is attested earliest in Spanish, where it was in place before 1600.

No word beginning with J is of Old English derivation. [OED]

English dictionaries did not distinguish words beginning in -i- and -j- until 19c., and -j- formerly was skipped when letters were used to express serial order.

In Latin texts printed in modern times, -j- often is used 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 has been 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 English words from Hebrew, -j- represents yodh, which was equivalent to English consonantal y (hence hallelujah) but many of the Hebrew names later were conformed in sound to the modern -j- (compare Jesus).

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Jabberwocky 

1871, 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.

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Jack 

masc. proper name, attested by 1218, probably via Anglo-French Jake, Jaikes, from Old French Jacques (which was a diminutive of Latin Jacobus; see Jacob), but in English the name always has been regarded as a familiar form of John, and some have argued that it is a native formation. In Middle English spelled Jakke, Jacke, etc., and pronounced as two syllables ("Jackie").

In England, Jack became a generic name applied familiarly or contemptuously to anybody (especially a young man of the lower classes) from late 14c. Later used especially of sailors (1650s; Jack-tar is from 1781); Jack-ashore (adj.) "drinking and in high spirits, recklessly spending" (1875) also is an image from sailors (1840 as a book title). In U.S., as a generic name addressed to an unknown stranger, attested from 1889. Every man Jack "everyone" is from 1812. Also see jack (n.).

Used in male personifications from 15c.; first record of jack-of-all-trades "person handy at any kind of work or business" is from 1610s; Jack Frost is from 1826; Jack-nasty "a sneak or sloven" is from 1833 (Jack-nasty-face, a sea-term for a common sailor, is from 1788). Jack Sprat for a small, light man is from 1560s (his opposite was Jack Weight). Jack-pudding "comical clown, buffoon" is from 1640s. Jack-Spaniard is from 1703 as a Spaniard, 1833 as "a hornet" in the West Indies. Other personifications listed in Farmer & Henley include jack-snip "a botching tailor," Jack-in-office "overbearing petty official" (1680s), Jack-on-both-sides "a neutral," Jack-out-of-doors "a vagrant" (1630s), jack-sauce "impudent fellow" (1590s).

The U.S. plant jack-in-the-pulpit (Indian turnip) is attested by 1833. Jack the Ripper was active in London 1888. The Scottish form is Jock (compare jockey (n.)). Alliterative coupling of Jack and Jill is from 15c. (Iakke and Gylle, Ienken and Iulyan). Jack Ketch for "hangman, executioner" (1670s) is said to be from the name of a public executioner in the time of James II (compare Derrick); it also was used as a verb meaning "to hang."

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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."
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Jack-o'-lantern (n.)
also jack-o-lantern, jack-a-lantern, jackolantern, 1660s, "night-watchman;" 1670s as a local name for a will-o-the-wisp (Latin ignis fatuus), mainly attested in East Anglia but also in southwestern England. Literally "Jack of (with) the lantern;" see Jack + lantern. The extension to carved pumpkin lanterns is attested by 1834 in American English.
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Jacksonian 
1824, of or in the character of U.S. politician Andrew Jackson (1767-1845). The surname is recorded from early 14c., literally "Jack's son, son of a man named Jack." Jacksonville, Florida, was renamed for him in 1822 from earlier Cowford, said to be an English translation of a native name wacca pilatka [Room].
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Jacob 

masc. proper name; 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; a supplanter" (Genesis xxv.26), 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 Genesis xxviii.12. In Spanish as Jago, Iago, also Diego; with alterations as Italian Giacomo, James, and (contracted) Spanish Jaime.

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Jacobin (n.)
early 14c., "Dominican friar," 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, for which see Jacob.

The Revolutionary extremists ("Society of the Friends of the Constitution") made their club headquarters there October 1789 and supported Robespierre during the Terror. They were suppressed along with him in November 1794 and many members executed. In English, the word quickly became a scare-word for the worst excesses of the French Revolution, and since 1793 it has been used generically and often inappropriately of allegedly radical politicians and reformers. Related: Jacobinism; Jacobinic; Jacobinical.
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Jacquerie (n.)

"French peasantry," 1520s, from French jacquerie "peasants or villeins collectively" (15c.), from Jacques, the proper name, which is used as Jack is used in English, in the sense of "any common fellow." So it also means "the rising of the northern French peasants against the nobles in 1357-8," from a French usage. Etymologically, Jacques is from Late Latin Iacobus (see Jacob).

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Jacuzzi (n.)

type of whirlpool bath, 1961, U.S. proprietary name, from Jacuzzi Brothers, then headquartered in California, who earlier made jet pumps for motorboats. The family immigrated from Friuli in northern Italy.

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