Ace in the hole
This idiom or expression means:
Big secret help.
A winning factor kept hidden.
She is our ace in the hole. With her at our side, I’m sure we’re going to win the game. But keep it to yourself for now.
The prosecutor had an ace in the hole: an eyewitness!
History:
May have its origin in the game of poker where you have an ace with the face down, until it’s time to show it.