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.