play trump card
play (one's) trump card
To use a particular stratagem or resource that will give one a distinct advantage over others, often by being held and then used at an opportune time. I have a feeling the prosecution still hasn't played their trump card in this trial—I'm expecting a big revelation during tomorrow's testimony. I was looking like I would get full custody of the kids, but then Mary played her trump card—pictures of me going in and out of various women's houses around the city.
play one's trump card
1. Lit. [in certain card games] to play a card that, according to the rules of the game, outranks certain other cards and is thus able to take any card of another suit. Bob played his trump card and ended the game as the winner.
2. Fig. to use a special trick; to use one's most powerful or effective strategy or device. I won't play my trump card until I have tried everything else. I thought that the whole situation was hopeless until Mary played her trump card and solved the whole problem.