an ace up your sleeve （美式英语：an ace in the hole）
秘密王牌、应急妙计、锦囊妙计
- Our superior technology is our ace in the hole.
我们超众的技术是我们的法宝。 - Unless you've got an ace up your sleeve, we are dished.
除非你有锦囊妙计，否则，我们是输定了。 - The mayor's popularity among elderly voters gives him an ace up his sleeve for the coming election.
市长在年长选民中的声望使他为即将到来的选举赢得了一张秘密王牌。
