1. One of the many sounds that retards make.
2. The sound you might make when imitating someone that just did something retarded (see #1).
3. What you might call someone that just did something really stupid.
*Joe and Tom play some random FPS, split-screen*
*Tom throws a grenade at his own feet, killing himself*
(#2) Joe: Durp Durp! Look at you trying to run, good job retard.
(#3) Joe: Tom, you are such a friggin' Durp Durp...