v. & n.
v. 1 tr. & intr. munch or chew noisily. 2 tr. (of a horse etc.) work (the bit) noisily between the teeth. 3 intr. fret with impatience (is champing to be away).
n. a chewing noise or motion. champ at the bit be restlessly impatient.
[ prob. imit. ]