A cheat, trick, or contrivance, I will not be fobbed off so; I will not be thus deceived with false pretences. The fob is also a small breeches pocket for holding a watch.
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.
Flymsey * FogNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes