A cheat, or trick. Do you think to fun me out of it? Do you think to cheat me?—Also the breech, perhaps from being the abbreviation of fundament. I’ll kick your fun. Cant.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Fumbler * To FunkNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes