Champ at the bit
Be restless and impatient to commence; especially during an unwelcome delay.
The verb 'champ' means 'to make a biting or chewing action with the jaws and teeth". Of course, it was horses that were first said to be 'champing at the bit'; the bit being the mouthpiece of a horse's bridle.
The earliest citation of 'champ at the bit' that I can find comes in the Reverend Charles Lucas's work Joseph, a religious poem, 1810:
Twelve beauteous steeds, of golden color and with golden manes, champ at the bit.
Despite the apparently American spelling of 'color', Lucas was English, being the curate of Avebury in Wiltshire in 1810.
There is another Americanisation connected with this phrase, that is, 'chomp at the bit'. This has become the more commonly used spelling in recent years, much to the disgust of purists. 'Chomp' began to replace 'champ' in the USA in the early 20th century. The first reference I can find to 'chomping at the bit' is from a recruitment advert in The Decatur Daily Review, April, 1920:
When the horses arc chomping at the bit and the "yellow legs" mount up and the troop rides forth, there is a thrill that no old cavalryman can ever forget.
Horses still champ/chomp at the bit of course and that literal usage is still commonplace amongst horsy folk. The figurative usage, which refers to someone who is impatient, but human rather than equine, began in the mid 19th century. An early example of that metaphorical usage was printed 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.