late 13c., from O.Fr. felon "evil-doer, scoundrel, traitor, rebel, the Devil" (9c.), from M.L. fellonem (nom. fello) "evil-doer," of uncertain origin, perhaps from Frank. *fillo, *filljo "person who whips or beats, scourger" (cf. O.H.G. fillen "to whip"); or from L. fel "gall, poison," on the notion of "one full of bitterness." Another theory (advanced by Professor R. Atkinson of Dublin) traces it to L. 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.
Fecund, which is etymologically connected to felon, female, and felix (happy), also means one who sucks and one who suckles, via the Proto-Indo European root dhe.