"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.
"fold of flesh under the jaw," 1590s, alteration of Middle English cholle "fold of flesh hanging from the neck or jaw, double chin" (c. 1300), which is perhaps from or related to Old English ceole "throat" (from PIE root *gwele- (3) "to swallow;" see gullet), but the phonetic development would be abnormal. Also see jowl (n.1).