trump

come up/turn up trumps (British & Australian)

to complete an activity successfully or to produce a good result, especially when you were not expected to. John's uncle came up trumps and found us a place to stay at the last minute.

See also: come, turn

a trump card

an advantage that makes you more likely to succeed than other people, especially something that other people do not know about. The fact that I had an Italian parent turned out to be my trump card when I applied for the job. Anthea was about to play her trump card - none of the money could be released without her signature.

See also: card