From the movie The Big Lebowski. The Dude is confused with The Big Lebowski.
Let me explain something to you. Um, I am not "Mr. Lebowski". You're Mr. Lebowski. I'm the Dude. So that's what you call me. You know, that or, uh, His Dudeness, or uh, Duder, or El Duderino if you're not into the whole brevity thing.