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.