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
Marianna	mer-ee-AN-ə (English), mar-ee-AN-ə (English), mahr-YAHN-nah (Polish)	Italian, English, Hungarian, Slovak, Polish, Greek
Caerwyn	-	Welsh
Damian	DAY-mee-ən (English), DAHM-yahn (Polish)	English, Polish, Dutch
Ros	RAHZ, RAHS	English
Marquis	mahr-KEE	African American
Iacomus	-	Late Roman