A lewd woman, or one that plays with her tail; also an impotent man, or an eunuch. Tag, rag, and bobtail; a mob of all sorts of low people. To shift one’s bob; to move off, or go away. To bear a bob; to join in chorus with any singers. Also a term used by the sellers of game, for a partridge.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Bob Stay * Body SnatchersNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes