I was curious--and, I think we've had this on here before, but. This is from the "other" OED (the Online Etymology Dict.):

flinch (v.)
1570s, from obsolete flecche "to bend, flinch," probably from O.Fr. flenchir "to bend," probably from Frank. *hlankjan (cf. M.H.G. linken, Ger. lenken "to bend, turn, lead"), from PIE base *kleng- "to bend, turn" (see flank). Related: Flinched; flinching. As a noun, from 1817.

Arrow fletches? (Hi, doc comfort)