also david, "crane-like structure on the side or stern of a vessel for suspending or lowering a boat," late 14c., daviot, apparently a use of the masc. proper name David on the pattern of applying common Christian names to useful devices (compare jack, jenny, jimmy).
"a buffoon; a zany; a jack-pudding" [Johnson], "One whose business it is to make sport for others by jokes and ridiculous posturing" [Century Dictionary], according to OED, in early use properly a mountebank's assistant, 1670s, from merry + masc. proper name Andrew, but there is no certain identification with an individual, and the name here may be generic.
by 1926, "mediocre prizefighter," of unknown origin, credited to U.S. sportswriter and Variety magazine staffer Jack "Con" Conway (1898-1928), who might at least have popularized it. Non-boxing sense of "average person" is from Joe Palooka, hero of Ham Fisher's boxing-themed comic strip, which debuted in 1930.
"depart quickly," often as an interjection, 1928, U.S. slang, either a shortened form of scramble (v.) or from German schramm, imperative singular of schrammen "depart," which is of uncertain origin. Said to be another coinage of U.S. sportswriter and Variety magazine staffer Jack "Con" Conway (1898-1928). Related: Scrammed; scramming.
also Jack Tar, "sailor," 1670s, probably a special use of tar (n.1), which stuff was a staple for waterproofing aboard old ships (knights of the tarbrush being a jocular phrase for "sailors"); or possibly a shortened form of tarpaulin, which was recorded as a nickname for a sailor in 1640s, from the tarpaulin garments they wore.