The tail. The croppen of the rotan; the tail of the cart. Croppen ken: the necessary-house. Cant.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Cropping Drums * CropsickNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes