1: Disgusting rag waved by Steelers fans at football games, tailgates, and business meetings. Generally left unwashed for "good luck."
2: A synonym for something that is utterly repulsive, idiotic, or represents a lower stratum in human evolution.
Guy 1: Dude, that girl is a major slew, I can't believe you hooked up with her.
Guy 2: I know, terrible towel of a decision. I better get tested asap.