A hidden advantage or resource kept in reserve until needed. The term comes from stud poker^{1, 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的意思是潛在的或者被隱藏有哪些好處。源自一種撲克游戲，玩家有一張牌正面朝下，假如它正好是一張A，那就對玩家有好處。Hole在這里僅僅表示隱藏的。 十九世紀(jì)美國西部用它來形容隱藏的武器。比如藏在肩袋中的手槍。到十九世紀(jì)二十年代，它日漸表示所有叫人驚喜有哪些好處或者殺手锏。

例：The prosecutor^{6 had an ace in the hole: an eyewitness7. 檢舉人有秘密武器（神秘王牌）：目擊者。}