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."/
See: HANG ON TO YOUR HAT.

[TahlilGaran] English Idioms Dictionary ▲