"jaw, jawbone," especially the underjaw, a late 16c. alteration of Middle English chawl (late 14c.), earlier chafle (c. 1200), from late Old English ceafl "jaw; cheek; jawbone; cheekbone," from Proto-Germanic *kefalaz (source of Middle High German kiver, German kiefer, Old Norse kjoptr "jaw," Danish kæft, Flemish kavel, Dutch kevel "gum"), from PIE *gep(h)- "jaw, mouth" (cognates: Old Irish gop, Irish gob "beak, mouth").
The spelling with j-, attested from c. 1400, is perhaps from influence of the synonymous Old French joue, which also was in Middle English (see jaw (n.)). This word and jowl (n.2) have influenced one another in form and sense. Middle English also had a jolle (late 14c.) meaning "the head," especially that of a fish, which might be from either or both nouns.