champ at the bit, to

To express impatience at delay, to be eager to get going. To champ has meant to bite, chew, or grind upon since the sixteenth century, although its precise origin is uncertain. The analogy of the cliché is to a racehorse chewing on the bit at the start of a race, anxious to be off. The term was still being used literally in the nineteenth century (“The very horses champed at their bits,” Sketch Book, Washington Irving, 1820) but began to be used figuratively by 1900.