ace in the hole：应急的法宝；备用的王牌，底牌；杀手锏

e.g. The twenty dollar bill I keep in my shoe is my ace in the hole.

我放在鞋子里的那张20美元的钞票是备来急用的。

e.g. Mary's beautiful singing voice was her ace in the hole in case everything else failed.

万般无奈时，玛丽那副美妙的歌喉可是她最后一张王牌。

e.b. The football team has a new play that they are keeping as an ace in the hole for the big game.

这支足球队准备在这次大赛上展示一种新打法，从而出奇制胜。