hang on to your hat

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.