Champ at the bit
To champ at the bit is to be eager and impatient to start something or other, and dates in this figurative sense from the mid-1600s. It obviously alludes to horses who will champ or bite at the bit in their eagerness to get going. Champ is an imitative word dating from c. 1530 that means to bite or chew vigorously. Chomp is the American equivalent and has largely taken over from champ since the early 20th century.