Let's get this one out of the way.
A trump card is "a decisive overriding factor or final resource." It refers to a card of any suit that ranks higher than any card of any of the other suits, and is used in many games, including euchre and spades. Since such cards are deployed strategically, the term suggests an option or weapon held in reserve.
The term finds itself in idioms that tie back to games, such as play one's trump card or have a trump card up one's sleeve.
For, after all, she was the mother here. The mother of the boys in question. She held the trump card, should she be forced to play it.
— Zadie Smith, White Teeth, 1999