felon

The adjective is derived from Middle English feloun, felun (“base, wicked; hostile; of an animal: dangerous; of words: angry, harsh, slanderous; of things: dangerous, deadly; false, fraudulent; unlucky”) [and other forms], from Old French felon (“bad, evil, immoral”) (compare fel (“evil; despicable, vile”)), from Early Medieval Latin fellōnem; further etymology uncertain.

Doublet of fell (“of a strong and cruel nature; fierce; grim; ruthless, savage”).

Etymology 1, adjective sense 3 (“obtained through a felony”) is derived from the noun.

The noun is derived from Middle English feloun, felun (“criminal, specifically one who has committed a felony, felon; cruel, hostile, violent, etc., person; deceiver; evildoer, monster, sinner; traitor; bold or fierce warrior; deceit, falseness; wickedness, wrongdoing; treachery”) [and other forms], from feloun, felun (adjective): see above.