"move up and down with a short, jerking motion," late 14c., probably connected to Middle English bobben "to strike in cruel jest, beat; fool, make a fool of, cheat, deceive" (early 14c.), which is perhaps from Old French bober "mock, deride," perhaps ultimately of echoic origin. Related: Bobbed; bobbing. The sense "snatch with the mouth something hanging or floating," as in bobbing for apples (or cherries), is recorded by 1799. To bob and weave in boxing is by 1928. Compare bob (n.2).
"short hair," 1680s; attested 1570s in sense of "a horse's tail cut short," from earlier bobbe "cluster" (as of leaves), mid-14c., a northern word, perhaps of Celtic origin (compare Irish baban "tassel, cluster," Gaelic babag).
The group of bob words in English is of obscure and mostly colloquial origin; some originally were perhaps vaguely imitative, but they have become more or less entangled and merged in form and sense. As a noun, it has been used over the years in various senses connected by the notion of "round, hanging mass," and of weights at the end of a fishing line (1610s), pendulum (1752) or plumb-line (1832). The hair sense was revived with a shift in women's styles starting in 1918 (when it was regarded as a sign of radicalism) and the modern noun meaning "a bobbed hair style" dates from 1920.
In the latter years of the decade [1920s] bobbed hair became almost universal among girls in their twenties, very common among women in their thirties and forties, and by no means rare among women of sixty .... Women universally adopted the small cloche hat which fitted tightly on the bobbed head, and the manufacturer of milliner's materials joined the hair-net manufacturer, the hair-pin manufacturer, and the cotton goods and woolen goods and corset manufacturers, among the ranks of depressed industries. [Frederick Lewis Allen, "Only Yesterday"]
slang word for "shilling," 1789, but the signification is unknown.
a familiar shortening and alteration of the masc. proper name Robert. British slang phrase Bob's your uncle "everything's all right" is attested by 1937. It seems to echo the old use noted in the 1725 "Canting Dictionary," which reports "Bob ... signifies Safety, ... as, It's all Bob, i. e. All is safe, the Bet is secured."