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

ace in the hole指的是争论，计划或者直到必要的时候才公布的事情。

An ace in the hole is any argument, plan or thing kept hidden until needed. It is used especially when it can turn failure into success.

它的意思是到需要时才公开的观点、计划等东西，特别用在转败为胜时。