A sharp, cunning fellow. Also an old term for a sword, probably a rusty one, or else from its being dyed red with blood; some say this name alluded to certain swords of remarkable good temper, or metal, marked with the figure of a fox, probably the sign, or rebus, of the maker.
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.
Fousil * Fox’s PawNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes