fretyoudaoicibaDictYouDict[fret 词源字典]
fret: English has three separate words fret. Fret ‘irritate, distress’ [OE] goes back to a prehistoric Germanic compound verb formed from the intensive prefix *fra- and the verb *etan (ancestor of English eat), which meant ‘eat up, devour’. Its modern Germanic descendants include German fressen ‘eat’ (used of animals). In Old English, it gave fretan, which also meant ‘devour’, but this literal meaning had died out by the early 15th century, leaving the figurative ‘gnaw at, worry, distress’. Fret ‘decorate with interlaced or pierced design’ [14] (now usually encountered only in fretted, fretwork, and fretsaw) comes from Old French freter, a derivative of frete ‘trellis, embossed or interlaced work’, whose origins are obscure.

Also lost in the mists of time are the antecedents of fret ‘ridge across the fingerboard of a guitar’ [16].
=> eat[fret etymology, fret origin, 英语词源]
fret (v.)youdaoicibaDictYouDict
Old English fretan "devour, feed upon, consume," from Proto-Germanic compound *fra-etan "to eat up," from *fra- "completely" (see *per- (1)) + *etan "to eat" (see eat). Cognates include Dutch vreton, Old High German freggan, 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.
fret (n.1)youdaoicibaDictYouDict
"ornamental interlaced pattern," late 14c., from Old French frete "interlaced work, trellis work," probably from Frankish *fetur or another Germanic source (cognates: Old English fetor, Old High German feggara "a fetter, shackle") perhaps from the notion of "decorative anklet," or of materials "bound" together.
fret (n.2)youdaoicibaDictYouDict
"ridge on the fingerboard of a guitar," c. 1500, of unknown origin, possibly from another sense of Old French frete "ring, ferule." Compare Middle English fret "a tie or lace" (early 14c.), freten (v.) "to bind, fasten" (mid-14c.).