#5: Ace in the Hole
ace in the hole
1 : an ace dealt face down to a player (as in stud poker) and not exposed until the showdown
2 : an effective and decisive argument or resource held in reserve
"Ace in the hole is a phrase that's both familiar and vague but its meaning becomes clear when you read the first definition (the one that talks about actual playing cards) along with the second more metaphorical one."