To vehemently maintain that one is awesome at something, despite evidence that the person actually really sucks at it.
The act of not having the sense to admitting one's deficiencies in something right after proving that one is deficient.
-"Dude you just scored an own goal!"
-"No man, I'm awesome. It was just the wind, I'll score the next goal."
-"Dude, don't pull a uni..."
If you act like a player in front of your friends, then freeze up when talking with a chick, then brag to your friends about what a player you are later that night then you just pulled a uni.