A hidden advantage or resource kept in reserve until needed. The term comes from stud poker1, where each player is dealt one card face down--the so-called hole card--and the rest face up. Should the hole card be an ace2, the player has a hidden advantage. Hole here simply means a hiding place. In the 19th-century American West, the expression was used to refer to a hidden weapon, such as a gun concealed3 in a shoulder holster. By the 1920s it had become a metaphor4 for any surprise advantage or leverage5.
Note:
ace in the hole的意思是潜在的或者被隐藏有哪些好处。源自一种扑克游戏，玩家有一张牌正面朝下，假如它正好是一张A，那就对玩家有好处。Hole在这里仅仅表示隐藏的。 十九世纪美国西部用它来形容隐藏的武器。比如藏在肩袋中的手枪。到十九世纪二十年代，它日渐表示所有叫人惊喜有哪些好处或者杀手锏。
例：The prosecutor6 had an ace in the hole: an eyewitness7. 检举人有秘密武器（神秘王牌）：目击者。