Green’s Dictionary of Slang

fun n.1

[SE fundament]

the buttocks, the backside.

[UK]B.E. Dict. Canting Crew n.p.: Fun [...] an Arse.
[UK]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.
[UK]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.
[UK]Bailey Universal Etym. Eng. Dict. [as cit. 1725].
[UK]Grose Classical Dict. of the Vulgar Tongue.
[UK]Lex. Balatronicum.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.

In compounds