Hidden advantage. Unseen asset. Ace cards in poker games are the highest winning card, so if you hide one in your sleeve while playing, you can, illegally and against good sportsmanship, possibly win the hand and the money. The phrase does not always suggest an illegal or immoral action, simply that one has another asset unknown to competitors. (Compare to Ace in the Hole, Edge or Card up your sleeve.)

“I know she has several boyfriends but I’m going to become her one and only because I have an ace up my sleeve because I’m going to gift  her with a cute puppy.”