"You canker blossom!" 3 Shakespearean Insults
late 13c., from Old French felon "evil-doer, scoundrel, traitor, rebel, the Devil" (9c.), from Medieval Latin fellonem (nominative fello) "evil-doer," of uncertain origin, perhaps from Frankish *fillo, *filljo "person who whips or beats, scourger" (cf. Old High German fillen "to whip"); or from Latin fel "gall, poison," on the notion of "one full of bitterness."
Another theory (advanced by Professor R. Atkinson of Dublin) traces it to Latin fellare "to suck" (see fecund), which had an obscene secondary meaning in classical Latin (well-known to readers of Martial and Catullus), which would make a felon etymologically a "cock-sucker." OED inclines toward the "gall" explanation, but finds Atkinson's "most plausible" of the others.
felon fel·on (fěl'ən)
A purulent infection or abscess involving the bulbous distal end of a finger. Also called whitlow.