FUN,
is also used for the Backside; as, I'll kick your Fun, i. e. I'll kick your Breech. Likewise for Game of Diversion; as We had rare Fun with him.
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Entry taken from 1736 Canting Dictionary, by Nathan Bailey.
The Vulgar Tongue originally collected by Grose, 1811.
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).