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1.
The guy with the best motor-skills and coordination under the influence of a lot of alcohol out of all the dudes you hang out with. You depend on this guy to to drive you home when all of you are wasted and you trust him with your life. He should be able to take like 20 brews and still drive you back to base without a fatal accident or getting pulled over.
Since we all planed on drinking we decided that Arlen would be the designated drunk driver because he drove back to the city that one night without dying after drinking 15 shots of vlady.
by Kurt Grendel January 29, 2008
 
2.
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.
by J Filthy Money June 14, 2010