In stud poker, each player is dealt one card face down,which was called a ____ (as referring to a hiding place) and the rest face up. So if you had X, you were at an advantage. 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. Now it is used as a metaphor for any surprise advantage or leverage.
What phrase does X refer to?