ace 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. My stamina is the ace up my sleeve for this race—the other runners don't stand a chance! The defense attorney waited for just the right time to play the ace up her sleeve—a new eye-witness. She's keeping all the details of their unscrupulous business practices as an ace in her sleeve should they ever try to fire her.