"jaw," 1570s, alteration of Middle English chawl (late 14c.), chavel (early 14c.), from Old English ceafl, from Proto-Germanic *kefalaz (cf. Middle High German kiver, German kiefer, Old Norse kjoptr "jaw," Danish kæft, Flemish kavel, Dutch kevel "gum"), from PIE *gep(h)- "jaw, mouth" (cf. Old Irish gop, Irish gob "beak, mouth"). The change from ch- to j- has not been explained.