have a card up your 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
Farlex Dictionary of Idioms.

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
Farlex Partner Idioms Dictionary
See also:
  • a card up (one's) sleeve
  • ace up (one's) sleeve
  • card up one's sleeve
  • have an ace up (one's) sleeve
  • have an ace up one’s sleeve
  • have an ace up your sleeve
  • have an ace/a trick up your sleeve
  • have up sleeve
  • have/keep something up your sleeve
  • trick up (one's) sleeve