trump card

[truhmp kahrd, truhmp kahrd]

noun

1. Cards.,  trump.

2. Informal.,  something that gives one person or group the advantage over another.

   The surprise witness was his trump card.

trump card

1. In general, something capable of making a decisive difference when used at the right moment; in certain card games, trump is the suit designated as having precedence over the others: “The prosecutor was about to win the case, when the defense lawyer produced her trump card: an eyewitness who testified that the accused was nowhere near the scene of the crime.”

Word History and Origins

Origin of trump card¹

First recorded in 1815–25

Idioms and Phrases

A key resource to gain an advantage at the opportune moment, as in That surprise witness was the defense's trump card, or She played her trump card, announcing that the Senator would speak. This expression transfers the trump card of games such as bridge, which can win over a card of another suit, to other kinds of advantage. [Early 1800s]