the.com/ace up the sleeve

means A hidden advantage or resource kept secret until the moment it's needed most.

from Pulled straight from card-cheating slang — literally hiding a winning card (an ace) up your sleeve to produce when the hand turns against you. The phrase surfaces in English in the 19th century, riding on the broader cultural image of the card sharp, and it stuck because the physical cheat — sleeve as secret pocket — made the metaphor of a concealed advantage instantly legible, honest gameplay or not.