A good job, or snack easily got: also shellfish growing at the bottoms of ships; a bird of the goose kind; an instrument like a pair of pincers, to fix on the noses of vicious horses whilst shoeing; a nick name for spectacles, and also for the gratuity given to grooms by the buyers and sellers of horses.
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.
Barnaby * Barrel FeverNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes