card up sleeve
a card up (one's) sleeve
A secret plan, idea, or advantage that can be utilized if and when it is required. A reference to cheating at a card game by hiding a favorable card up one's sleeve. She has a card up her sleeve if they ever try to fire her, since she knows about the boss's unscrupulous business practices.
card up (one's) sleeve
A secret resource or plan held in reserve: a tough negotiator who had a number of cards up his sleeve.