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.

ace in the hole (plural aces in the hole)

(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.

ace up one's sleeve — see ace up one's sleeve

English