"jaws, sides of the face," c. 1500, perhaps a variant of chaps (n.2) in the same sense, which is of unknown origin.
Entries linking to chops
chaps n.2
"jaws, cheeks," from chap (n.), 1550s, which is of unknown origin. Hence, chap-fallen "with the lower jaw hanging down" (1590s), hence, figuratively, "dejected, disspirited" (c. 1600).