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的好处。来源于一种扑克游戏，玩家有一张牌正面朝下，如果它正好是一张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。例如藏在肩袋中的手枪。到十九世纪二十年代，它渐渐表示所有让人惊喜的好处或者杀手锏。

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。