When you pull your dick out of a girl's ass(or guys) and the tip of your dick is dirty. Since the tip of your dick looks like Darth Vador's helmet, it is hence called a "Dirty Darth."
Guy1: So did you get some analriffic
action last night?
Guy2: Yeah, I even got a dirty darth, but it's okay, I wiped it off on her upper lip giving her a dirty sanchez
when at the end of anal sex, the tip of the penis is covered in feces.The tip of the penis looks like the head of Darth Vader thus the name, dirty darth.
man 1: hey, did you have sex with her last night?
man 2: yeah, but we did anal. It was ok but by the end I got a dirty darth.