a hole card
1. In poker, a card dealt face down to a player. It is common in stud poker for the player to receive a number of hole cards.
2. A secret advantage. Just when everyone thought Randy was going to lose the competition, he used his hole card and pulled into first place.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
a hole cardmainly AMERICAN
A hole card is an advantage that you keep secret until you are ready to use it. The fact that I knew where she was and had in my possession a boxful of evidence were my only two remaining hole cards. Note: In five card `stud' poker, the `hole' card is the only card which is dealt to you face down so that the other players cannot see it.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012