[hang on to your hat] or [hold on to your hat] or [hold your hat] {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."/