According to the Mug My Kids Made Me

Dude: So I'll probably drive everyone home from the bar tonight.
Girlfriend: Really? Awesome!
Dude: Yeah, we go out all the time, and I'm almost always the dd.
Girlfriend: Wow, that's really great of you.
Dude: Yeah, I'm really good at driving drunk. I'm like the world's best drunk driver!

–Terminal 5, JFK

Overheard by: Don't think that's what DD stands for