"be peevish or worried," early 12c., from Old English fretan "eat, devour" (in Old English used of monsters and Vikings; in Middle English used of animals' eating), from Proto-Germanic compound *fra- "for-" + *etan "to eat" (cf. Dutch vreton, Old High German freggan, German fressen, Gothic fraitan). Transitive sense of "eat away" is from late 12c. Figurative sense of "irritate, worry, eat one's heart out" is c.1200. Modern German still distinguishes essen for humans and fressen for animals. Related: Fretted; fretting. As a noun, from early 15c.
"ornamental interlaced pattern," late 14c., from Old French frete "interlaced work, trellis work," probably from Frankish *fetur or another Germanic source (cf. Old English fetor, Old High German feggara "fetter") perhaps from notion of "decorative anklet," or of materials "bound" together. The other noun, "ridge on the fingerboard of a guitar," is c.1500 of unknown origin but possibly another sense of Old French frete.