FUN,
a Cheat, or slippery Trick; What do you fun me? Do you think to sharp or trick me? He put the fun upon the Cull, he sharped the Fellow.
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Entry taken from 1736 Canting Dictionary, by Nathan Bailey.
Fun in the 1811 Vulgar Tongue
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).