The origin of "Das" stems from the expression "that's a good one." It's commonly used in response to someone saying something extremely stupid that is complete nonsense. Technically, Das means "that's" which is a shortened version of the above expression.
1. Some cocky mofo: "I'm going to win the Heisman trophy next year as a freshman."
His friend: "That's a complete fuckin DAS!"
2. Same cocky mofo: "I've got a huge dick"
Everyone within earshot: "Ummm..DAS?!"
3. Cocky mofo's sister: "My big brother is the coolest!"