Definitions for trump card
Random House Webster's College Dictionary
Ref: trump1 (def. 1a). 1 1 1
something that gives a person or group a decisive or winning advantage.
Origin of trump card:
trump, trump card(noun)
a playing card in the suit that has been declared trumps
"the ace of trumps is a sure winner"
A playing card, of the trump suite that is held in reserve until needed to win a trick
Something used to obtain an advantage, sometimes unscrupulously