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
Mus'ad	-	Arabic
Taylan	-	Turkish
ZacarÍAs	thah-kah-REE-ahs (Spanish), sah-kah-REE-ahs (Latin American Spanish)	Spanish
Nine	NEE-nə	Frisian
Dechen	-	Tibetan, Bhutanese
Parvaneh	-	Persian