He knows something about the boss, which is his ace in the hole.

他对老板有所了解，这是他的王牌。

youdao

Our superior technology is our ace in the hole.

我们的优越技术是我们手中的王牌。

《柯林斯英汉双解大词典》

That spare can of gas in the trunk was our ace in the hole.

后背箱里那罐剩余的汽油是我们最后的储备。

youdao

For example, you hear the expression, ace in the hole, used by many people who would never think of going near a poker table.

VOA: special.2009.11.22

An ace in the hole is an argument, plan or thing kept hidden until needed.

VOA: special.2009.11.22

Often,the ace in the hole wins the game.

VOA: special.2009.11.22

Like any good gambler though, opponents of this critical resource development have an ace in the hole.

FORBES: EPA is Obama's Offshore Drilling Ace in the Hole

AIPAC's ace in the hole is the idea that it represents Jewish interests in a country that is generally philo-Semitic.

ECONOMIST: Lexington

Even without nuclear weapons, North Korea has an ace in the hole.

NPR: Despite Threats, Risks Temper Korea War Tensions