trump card

plural trump cards

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

   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.’

   2012, Zadie Smith, NW, London: Penguin Books, published 2013, →ISBN, page 160

   1 more example

   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?

   c. 1921 (date written), Karel Čapek, translated by Paul Selver, R.U.R. (Rossum’s Universal Robots): A Fantastic Melodrama […], Garden City, N.Y.: Doubleday, Page & Company, published 1923, →OCLC, Act 2