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.

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