YOUR BARK IS WORSE THAN YOUR BITE. "Mom said she would take away my computer but her bark is worse than her bite."
One's words are stronger than one's actions.
GET OFF YOUR HIGH HORSE "My boss is so conceited, I wish he would get off his high horse."
Become humble or more modest.
HARD NUT TO CRACK "This math problem is giving me trouble. It's a hard nut to crack."
Someone or something that is difficult to understand.
BACK THE WRONG HORSE "We voted for Mitt Romney in the election against Obama, but it looks like we were backing the wrong horse."
To support something that is unsuccessful.
WHEN THE CATS AWAY, THE MICE WILL PLAY I never, ever leave my team without supervision. Unfortunately, while the cat's away the mice will play.
This means that people usually behave differently when others are no watching them
SHAKE A LEG. Grandma told the boys to shake a leg and encouraged them to get a jumpstart on their chores.
This means to hurry up.