trump card

Noun

trump card (plural trump cards)

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