{v. phr.}, {informal} 1. Watch out; be prepared. - Used as a command, usually to warn of an unexpected action. * /"Hold on to your hat," said Jim as he stepped on the gas and the car shot forward./ 2. Get ready for a surprise. - Used as a command, usually to warn of unexpected news. * /"Hold on to your hat," said Mary. "Jim asked me to marry him."/
See: HANG ON TO YOUR HAT.