A stud poker
card dealt face down and hidden from the view of the other players is called a
hole card. An ace is the highest hole card possible, often making a
winning hand for the player holding it. Thus from this poker term came the
expression an ace in the hole for "any hidden advantage, something held
hidden in reserve until it is needed to win." The term probably dates back over
a century, and was first recorded in Collier's Magazine in 1922: "I got
a millionaire for an ace in the hole." Hole card is a synonym.