felonyoudaoicibaDictYouDict[felon 词源字典]
felon: [13] Medieval Latin fellō (a word of uncertain origin, sometimes referred to Latin fel ‘gall, poison’) meant ‘evil-doer’. Its nominative form gave English the adjective fell ‘fierce, lethal’, via Old French fel, while its stem form, fellōn-, passed into English through Old French felon. The derivative felony [13] comes from Old French felonie.
=> fell[felon etymology, felon origin, 英语词源]
felon (n.)youdaoicibaDictYouDict
c. 1300, "one who deceives or commits treason; one who is wicked or evil; evil-doer," used of Lucifer and Herod, from Old French felon "evil-doer, scoundrel, traitor, rebel, oath-breaker, the Devil" (9c.), from Medieval Latin fellonem (nominative fello) "evil-doer," which is of uncertain origin, perhaps from Frankish *fillo, *filljo "person who whips or beats, scourger" (source of Old High German fillen "to whip"); or from Latin fel "gall, poison," on the notion of "one full of bitterness." Celtic origins also have been proposed.

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.

Also by c. 1300 in English in a general legal sense "criminal; one who has committed a felony," however that was defined. Century Dictionary notes, "the term is not applicable after legal punishment has been completed." In Middle English it also was an adjective, "traitorous, wicked, malignant." Australian official James Mudie (1837), coined felonry "as the appellative of an order or class of persons in New South Wales,--an order which happily exists in no other country in the world."