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general::
Phrase(s): 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.
McGrawhill's American Idioms And Phrasal Verbs