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.
FOX
Meaning of FOX in English
Slang English vocab. Английский сленговый словарь. 2012