The act of telling a friend's biggest secret to a bunch of people to purposefully embarrass them. Or, saying the one phrase that could embarrass someone at the current moment. Another way to say "BURN!"
Carl: Well I saw you macking on Lisa.
Greg: I was only macking on Lisa because you were throwing up in her mom's vase and I was trying to distract her.
Carl: Way to pull out the Ace Card Greg.
(Tom witnessing a fight between Jan and Dee)
Dee: You never keep your side of the room clean.
Jan: Because I cant spend that much time in there with all you Hanson Posters all over.
Tom: Oh, ACE CARD!