Definition of "trump card" [trump card]

• Games A card in the trump suit, held in reserve for winning a trick. (noun)
• A key resource to be used at an opportune moment; a trump: "[They] seem determined to use the agreement as a trump card to obtain ... all the advantages they feel they deserve” ( Christian Science Monitor). (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "Joan Durbeyfield always managed to find consolation somewhere: 'Well, as one of the genuine stock, she ought to make her way with' en, if she plays her trump card aright."
• "She had always regarded “Ave Maria” as the Church’s trump card at funerals, but resisted tears by concentrating on the goings-on at the altar."