The phrase “ace in the hole” means: Something of especial effectiveness held in reserve or undisclosed; something kept up one’s sleeve.
The expression derives from the game of stud poker.
In this game the first round of cards in each deal is dealt face down, and each such card remains undisclosed to all but its holder until the end of the hand.
The next round is dealt face up, and that player with an exposed card of highest value may open the betting (or he may throw down his cards and pass the betting to the holder of the next best card).
Bets may be called or raised, or a player may pass out, but when the bets are even among the remaining players the dealer deals the third round, also face up.
Betting is resumed as before, the player with exposed cards of highest value having the privilege of opening.
When betting is again even, the fourth round is dealt, again face up, and betting is again resumed as before. Upon the fifth and final round, also face up, each player then knows the full value of his own hand, his “hole card,” the one undisclosed, plus the cards that have been exposed.
After final bets are completed each player exposes his card “in the hole” and the best hand wins.
Since aces are cards of highest value, that player with one, two, or three exposed aces and “an ace in the hole” has a card of especial effectiveness.