have a card up (one's) sleeve

have a card up (one's) sleeve

To have 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.
See also: card, have, sleeve, up

have a card up your sleeve

have a plan or asset that is kept secret until it is needed. British
See also: card, have, sleeve, up