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."
As a class of crime in common law, also from c. 1300, from Anglo-French. The exact definition changed over time and place, and even the distinction from misdemeanor or trespass is not always observed. In old use often a crime involving forfeiture of lands, goods, or a fee or a crime punishable by death. Variously used in the U.S.; often the sense is "crime punishable by death or imprisonment in a state penitentiary."
mid-15c., malefactour, "a law-breaker, a criminal, a felon," from Latin malefactor, agent noun from past participle stem of malefacere "to do evil," from male "badly" (see mal-) + facere "to do, make" (from PIE root *dhe- "to set, put"). From late 15c. in the now-obsolete sense of "one who does evil or injury to another" (the opposite of benefactor).
Most Indo-European words for "personal enemy" cover also "enemy in war," but certain languages have special terms for the latter, such as Greek polemioi (distinct from ekhthroi), Latin hostis, originally "stranger" (distinct from inimicus), Russian neprijatel' (distinct from vrag). Russian vrag (Old Church Slavonic vragu) is cognate with Lithuanian vargas "misery" (see urge (v.)), and probably is related to Proto-Germanic *wargoz, source of Old Norse vargr "outlaw," hence "wolf;" Icelandic vargur "fox;" Old English wearg "criminal, felon;" which likely were the inspirations for J.R.R. Tolkien's warg as the name of a kind of large ferocious wolf in "The Hobbit" (1937) and "Lord of the Rings." Related: Enemies.
Poisonous "sneak snakes" (because unlike the rattlesnake they strike without previous movement or warning), hence the figurative use in reference to hidden danger or secret hostility.
The copper-head, though smaller, was much more feared. The rattle-snake was larger, sooner seen, and a true southerner, always living up to the laws of honor. He would not bite without provocation, and by his rattles gave the challenge in an honorable way. Instead of this well-bred warfare, the copper-head is a wrathy little felon, whose ire is always up, and he will make at the hand or the foot in the leaves or grass, before he is seen, and his bite is as poisonous as that of his brother of the larger fang. The young men tested his temper, and found that in his wrath he would bite a red hot coal. [Henry Howe, "Historical Collections of Ohio," 1854]
Specifically in reference to Northerners suspected of sympathizing with the Southern rebellion, the name is said to have been first used in Greeley's New York "Tribune," July 20, 1861. Charles H. Coleman, "The Use of the Term 'Copperhead' During the Civil War" ["Mississippi Valley Historical Review" 25 (1938), p.263] traces it to an anonymous letter against Ohio anti-war Democrats in the Cincinnati "Commercial" newspaper in the summer of 1861. It seems not to have been in widespread use until summer 1862. Related: Copperheadism.