Anytime in a game when points don't really make a difference in the outcome of a game. It could be when a team is dominating another by so many points, they bring in the 2nd or 3rd stringers and they continue the ass-whooping. Another example would be a team that is getting their ass kicked real bad, but finally score. It's garbage time because the winning team has practically given up due to the spread being so huge.
Joe: "Man, why is Matt Cassel fist pumping after that TD?"
Greg: "I don't know, it's garbage time." "The Bills have already put this one away...in fact it was over in the 2nd quarter."