A tradesman who sells goods to young unthrift heirs, at excessive rates, and then continually duns them for the debt. To ferret; to search out or expel any one from his hiding-place, as a ferret drives out rabbits; also to cheat. Ferret-eyed; red-eyed: ferrets have red eyes.
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.
Ferrara * FetchNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes