the ace in your hand
the ace in (one's) hand
A major advantage that one keeps in reserve until an ideal time. The phrase originated in poker, in which an ace is the most valuable card. His embarrassing secret is the ace in my hand, and I plan to reveal it to everyone the next time he mocks me publicly. The country's supreme dominance in the manufacturing sector will forever be the ace in its hand when dealing with other nations on foreign-policy issues.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
the ace in your handBRITISH
The ace in your hand is something which you can use to gain an advantage when you need it. Note: In many card games, the ace is the card with the highest score. In politics, you have to convince your opponent that you have the ace in your hand.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012