The trump card is the winning card. If you play a trump card during a game of Spades, it means that you take the whole trick — in other words, you'll pick up every card on the table.
play one's trump card
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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.
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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.
trump card
The final resort but often the easiest way out.
You're most powerful option in succeeding at anything in life.
Unfortunately, once used, the trump card can rarely be used on the same person again. If reused it will rarely be as powerful.