ace in the hole

From stud poker in which there is one unknown card called the "hole card", referring to the optimum "hole card" being an ace.

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ace in the hole (plural aces in the hole)

1. (US, idiomatic, informal) Synonym of ace up one's sleeve

   Our ace in the hole left our opponents stupefied; it isn't every day that an NBA star plays street basketball.

   • 2008 July 14, Christopher Nolan, Jonathan Nolan, The Dark Knight, spoken by The Joker (Heath Ledger), Warner Bros. Pictures, DC Comics, Legendary Pictures:

     You didn't think I'd risk losing the battle for Gotham's soul in a fistfight with you? No. You need an ace in the hole. Mine's Harvey.