Green’s Dictionary of Slang

fun n.2

a cheat, a trick.

[UK]B.E. Dict. Canting Crew n.p.: Fun c. a Cheat, or slippery Trick.
[UK]New Canting Dict. [as cit. c.1698].
[UK]Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
[UK]Grose Classical Dict. of the Vulgar Tongue.
[UK]Lex. Balatronicum.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.
[UK]Marshall ‘Nobbled’ ‘Pomes’ from the Pink ’Un 115: I waited to pipe off the fun [F&H].