Green’s Dictionary of Slang

croppen n.

of an animal or a vehicle, the tail.

1707		J. Shirley Triumph of Wit 194: The Prancer drew the Quere Cove at the cropping of the Rottam through the Rum pads of the Rum-vill, and was flogg’d by the Nubbing-cove. [The Rogue was dragged at the Cart’s Arse through the chief Streets of London, and whipp’d by the Hangman].
1725		New Canting Dict. n.p.: croppin the Tail of any Thing; as, The Croppin of the Rotan; The Tail of the Cart.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. 1725].
c.1750–1882		B.M. Carew Life and Adventures.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1809		G. Andrewes Dict. Sl. and Cant n.p.: cropping the tail.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1835		G. Kent Modern Flash Dict.
1848		Flash Dict. in Sinks of London Laid Open.

In compounds