Fig.: to show impatience for being held back; to be eager to start

My dog was champing at the bit because he "knew" it was time to go to the park

Il mio cane mordeva il freno perché "sapeva" che era ora di andare al parco

Michael was champing at the bit to leave home and reach his volleyball team, but mom's car wasn't in sight yet

Michele mordeva il freno per uscire e raggiungere la sua squadra di pallavolo, ma la macchina di mamma non si vedeva ancora

Originally it refers to a race-horse champing at its bit while waiting impatiently to be off and run