Have a card up your sleeve
Definition 1 of 1
Pro Tip 1/3Metaphoric Use
The phrase is often used metaphorically rather than literally, referring to any unseen advantage. 
Even with odds against us, we still have a card up our sleeve.
Pro Tip 2/3Strategic Surprise
Use this phrase to imply a strategic surprise that can turn a situation around unexpectedly. 
They thought they had won, but we had a card up our sleeve.
Pro Tip 3/3Literal Origin
Originally from card games where a player might cheat by hiding a card, now means any secret strategy. 
This idiom comes from players who had a card up their sleeve to cheat in a game.