Old English fretan "devour, feed upon, consume," from Proto-Germanic compound *fra-etan "to eat up," from *fra- "completely" (from PIE root *per- (1) "forward," hence "through") + *etan "to eat" (from PIE root *ed- "to eat"). Cognates include Dutch vreten, Old High German frezzan, German fressen, Gothic fraitan.
Used of monsters and Vikings; in Middle English used of animals' eating. Notion of "wear away by rubbing or scraping" (c. 1200) might have come to this word by sound-association with Anglo-French forms of Old French froter "to rub, wipe; beat, thrash," which is from Latin fricare "to rub" (see friction). Figurative use is from c. 1200, of emotions, sins, vices, etc., "to worry, consume, vex" someone or someone's heart or mind, from either the "eating" or the "rubbing" sense. Intransitive sense "be worried, vex oneself" is by 1550s. Modern German still distinguishes essen for humans and fressen for animals. Related: Fretted; fretting. As a noun, early 15c., "a gnawing," also "the wearing effect" of awareness of wrongdoing, fear, etc.