ace in the hole

A hidden advantage or resource kept in reserve until needed, as in The prosecutor had an ace in the hole: an eyewitness. The term comes from stud poker, where each player is dealt one card face down—the so-called hole card—and the rest face up. Should the hole card be an ace, the player has a hidden advantage. Hole here simply means “a hiding place.” In the 19th-century American West, the expression was used to refer to a hidden weapon, such as a gun concealed in a shoulder holster. By the 1920s it had become a metaphor for any surprise advantage or leverage.

A hidden advantage or resource kept in reserve until needed: “The coach was certain that his new trick play would turn out to be his ace in the hole.” This term comes from the game of stud poker, in which one or more cards are turned face down, or “in the hole,” as bets are placed. The ace is the card with the highest value.