At the Kentucky Derby just before the race, there are many horses champing (not chomping) at the bit. Champing means chewing or grinding noisily, so a horse champing at the bit is a horse worrying the bit in his mouth because he is anxious for the race to start.
This idiom or phrase has been around at least since 1577, and is now usually applied to humans who are extremely eager to do something. But most people say chomping at the bit … probably because champing is such an uncommon verb. The British blame this on the Americans, who apparently began substituting “chomp” for “champ” in a big way back in the 1930s. Some writers claim it began even earlier than that, back in the 1800s.
Even though it’s not considered correct, a quick Google search shows that chomp at the bit is twice as common as the “champ” phrase, and a recent survey found that in American published books and newspapers chomped is outrunning champed in this linguistic race. Which word is going to win? Ultimately, it doesn’t really matter as many dictionaries accept both versions as correct, although we still prefer the older champ.
champ at the bit (phrase): to be impatient, eager to start