ace up (one's) sleeve

A secret advantage. 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.

Common Names:

Name	Pronounced	Usage
Naida	-	Croatian
Divya	-	Indian, Hindi, Marathi, Kannada, Tamil, Telugu, Malayalam
Balendin	-	Basque
Alvin	AL-vin	English
Atarah	AT-ə-rə (English)	Biblical, Hebrew, Biblical Hebrew
Royston	ROIS-tən	English (Rare)