an ace in the hole

• 备用的王牌；杀手锏；密藏的法宝

an ace in the hole的用法和样例：

例句

1. His strength in a crisis is an ace in the hole.
   临危不惧是他的看家本领。
2. But next year's national elections could be the mayor's ace in the hole.
   可是明年的全国大选可能是市长的杀手锏。