person who, in the case that everyone (including them) in the group is wasted to the point that they can't drive, has to drive everybody home, even though they're also wasted. Usually can drive really well when drunk. From "The Hangover"
Phil: Come on, just to Barstow! Everyone's passing us!
Doug: No, I promised I would be the only one driving. And besides, you're drinking.
Phil: Oh what are you now, a cop? You know I drive great when I'm drunk!
Stu: That's very true. If you'll remember, Phil was our designated drunk driver in college.