1
general::
Phrase(s): ace in the hole [and] someone’s ace in the hole
Fig. something important held in reserve. • The twenty-dollar bill I keep in my shoe is my ace in the hole., Phrase(s): someone’s ace in the hole
Go to ace in the hole.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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.
American Heritage Idioms