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.

Oh, Charles! …wait …what?
Gays are evil because they suck cock? Go on…
How could you not mention “fellatio?”
If this turns into a bunch of prison rape jokes, I will be disappointed but unsurprised.
@ 4, you’ll be partly responsible, then, for explicitly bringing it up. The mind that finds that stuff funny isn’t the kind that can find its own way there from what Brendan posted.
More lingustics posts, please.
When I got to the age when I wanted to, cock sucking was still a felony in Washington.The wording of the law burned itself into my tender 15 year old memory on first reading.
“It shall be unlawful to carnally know any other person, bird, animal or fish by mouth or anus. Punishment shall be 20 years in the state penitentiary.
That’s a little out of character for Fnarf, Matt. I would have expected something more along the lines of a music reference. Felonious Monk, maybe?
Don’t get too excited (I can tell it’s too late) about all these “suck” notions.
First, OED says of “felon” that there are many conjectured etymologies, and lays out the one it finds probable. Then it lists the ones it doesn’t find probable. OED finds Atkinson’s notion is “perhaps” the most plausible of those. Hardly a ringing endorsement.
(As an aside, Atkinson, an Englishman appointed a Trinity professor in the late 1800s, inveighed against the teaching of Irish sagas on the grounds all of them were “indecent”, and let it be known he discouraged others from speaking Gaelic in his presence.)
As for “fecund”, though it stems from a root whose many meanings include “one who sucks” or “one who suckles”, that does not make it remotely accurate to say either of those is what fecund *means*.
y’all so smart
Wasn’t felon also used for an infected pimple in the language of the late 18th Century?
@4, there was this black guy, a Jew and a Catholic who went to prison together…
What Gloomy Gus says. The most plausible etymology of “felon” to my mind is via French via Frankish, not Latin, and connects it to the same Germanic root as English “to fell.”
The “suck” connection isn’t through Latin specifically, either. It’s through Indo-European, which was likely spoken 6,000 years ago.
Felon used to be farmer slang for mastitis.