To show impatience or burst with energy to get going.
This phrase refers to the tendency of some horses, when impatient or nervous, and especially if being held back by their riders, to chew on the bit, often salivating excessively. This behavior may be accompanied by head-tossing or pawing at the ground. Because such behavior was most often seen in racehorses eager to begin a race, the term has become popular in everyday speech to refer to a person who is impatient to get started on something.
The verb ‘champ’ means ‘to make a biting or chewing action with the jaws and teeth.’ A bit is part of the apparatus that goes in the horse’s mouth and connects to the bridle and reins so the horse can be controlled by its rider. The bit fits into a toothless ridge of the horse’s mouth, so the horse never really bites the bit. But it can grind its teeth or jaw against the bit, which means that the horse is either nervous, or really excited about racing.
The earliest citation of ‘champ at the bit’ appears to be in the Reverend Charles Lucas’s work Joseph, a religious poem published in 1810.
The phrase is more commonly spelled “chomping at the bit” these days. “Chomp” began to replace “champ” in the USA in the early 20th century.
Horses still champ/chomp at the bit and that literal usage is still commonplace among people interested in horses. The figurative usage, which refers to a human who is impatient, began in the mid 19th century. An early example is found in the Ohio newspaper the Newark Daily Advocate, 1885:
‘Little breeches’ has been tramping down all the tall timber in his vicinity and champing at the bit tremendously, in his impatience to tackle Gov. Hoadley in a political discussion.
There are, of course, opposing opinions on the use of champing and chomping.
Webster’s says “champ at the bit” is to “show impatience at restraint; be restless.” It comes from something said about horses when they bite their bits “repeatedly and restlessly.” They “champ.” But, Webster’s does add that “chomp at the bit” is a variation.
The AP says “champ at the bit” is “the original and better form.”
William Safire wrote, some 31 years ago, that “to spell it champing at the bit when most people would say chomping at the bit is to slavishly follow outdated dictionary preferences.”
Obviously you can use whichever one you prefer. Since nobody can agree, I’ll probably make use of both.