Thought you might get a kick out of this - it's from the
Online Etymological Dictionary
fret (v.) - O.E. fretan "eat, devour" (in O.E., used of monsters and Vikings; in M.E., used of animals eating), from P.Gmc. compound *fra- "for-" + *etan "to eat" (cf. Du. vreton, O.H.G. freggan, Ger. fressen, Goth. fraitan). Figurative sense of "irritate, worry, eat one's heart out" is c.1200. Modern Ger. still distinguishes essen for humans and fressen for animals.
fret (n.) - "ornamental interlaced pattern," c.1386, from O.Fr. frete "interlaced work, trellis work," probably from Frank. *fetur (cf. O.E. fetor, O.H.G. 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 O.Fr. frete.
No mention of fretsaw/fretwork though.
cheers
Val