This was taken from this web page
www.langston.com/English/Colloquialisms
Champing at the bit
If someone is eager or anxious to do something, they are said to be champing at the bit, (not chomping at the bit. nor chomping on the bit).
CHAMPING: Repetitious, strong opening and closing action of the mouth which
produces sounds when the teeth hit together. Champing in swine may be a
threat signal, but also is performed by boars during courtship and
mating. Definition from Hurnik et al., 1995.
champ
v. tr. - To bite or chew upon noisily.
v. intr. - To work the jaws and teeth vigorously.
Idiom: - champ at the bit
To show impatience at being held back or delayed.