Felonious comes from felony + -ous, ultimately from Old French felon ('wicked, a wicked person'), from Medieval Latin fello, of uncertain origin, perhaps from Frankish or Latin fel ('gall, bile'). Morphologically, the account is both compositional and suffixal: -ous marks the visible structure by which the older base becomes the modern form. In present use, it is used as an adjective meaning "involving or constituting a serious crime; criminal", so the historical material remains connected with the current sense rather than standing as bare ancestry. The wider English family includes derivatives such as feloniously and feloniousness and related forms such as felon and felony, showing how the same historical material continues to organize neighboring vocabulary. The named historical forms include Old French felon, keeping the history tied to concrete forms rather than a loose resemblance.