an ace up your sleeve 秘密王牌；锦囊妙计

【例句】
Never fear! I have an ace up my sleeve.
别害怕！ 我有锦囊妙计。

The defense attorney waited for just the right time to play the ace up her sleeve—a new eye-witness.
辩护律师等待正确的时机出他的锦囊妙计，一个新的目击证人。