What is "champing at the bit"?

You've probably heard this saying, referring to being impatient and ready to go to do something, or having too much energy. It's obvious horse-related, so where does it come from?

It comes from the fact that some horses, when they're having to wait to do something or run will chew on the bit in an exaggerated way - "champing" is also spelled "chomping." An excited horse may also paw on the ground and throw his head around. This is often seen in racehorses during flag starts or in barrel racers in the chute - horses that are bred and trained to run and just want to get on with their job.