trump card

Definitions

1. A playing card, of the trump suit that is held in reserve until needed to win a trick.
2. Something used to obtain an advantage, sometimes unscrupulously.
   figuratively

Equivalents

Examples

“Domin The secret of their manufacture⟳. Old Rossum's manuscript. As soon as they found out that they couldn't make⟳ themselves they'd be on their knees to us. Dr. Gall Madame Domin, that was our trump card. I never had the least fear that the Robots would win⟳. How could they against people like⟳ us?”

“Desperate now to leave⟳, Felix played what he believed was his trump card. ‘You’re forty-whatever. Look⟳ at you. You’re still living like⟳ this. I want⟳ to have⟳ kids. I want⟳ to get⟳ on with my life.’”

CEFR level