c. 1300, "treachery, betrayal; deceit; villainy, wickedness, sin, crime; violent temper, wrath; ruthlessness; evil intention," from Old French felonie (12c.) "wickedness, evil, treachery, perfidy, crime, cruelty, sin," from Gallo-Roman *fellonia, from fellonem "evil-doer" (see felon).
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."