Green’s Dictionary of Slang

fun n.¹

the buttocks, the backside.

c.1698		B.E. Dict. Canting Crew n.p.: Fun [...] an Arse.
1709		N. Ward Compleat and Humorous Account of Remarkable Clubs (1756) 100: Loudly repeating some new Verses, hammer’d out of his dull Noddle for next Bumfodder Bill, that it might first make People laugh till they were ready to bedung themselves and do them the Service of a Paper-Muckender, to mundify their Funs.
1725		New Canting Dict. n.p.: fun is also used for the Backside; as, I’ll kick your Fun; i.e. I’ll kick your Breech.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. 1725].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.

In compounds

fun thruster (n.)

a sodomite.

c.1786

Grose Classical Dict. of the Vulgar Tongue ms. additions .