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."