have an ace up your sleeve
have an ace 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. I have an ace up my sleeve for this race—my stamina. The other runners don't stand a chance! I hear that the defense attorney has an ace up her sleeve—a new eye-witness.
have an ace up your sleevehave an effective resource or piece of information kept hidden until it is necessary to use it; have a secret advantage.
The ace is the highest playing card in its suit in many card games, so a cheating player might well hide one to use against an unwary opponent. A North American variant is an ace in the hole . The next two idioms are also based on this meaning of ace .