used primarily in Wales, refers to a guy that thinks he is very good at everything but in reality is average at best. Constantly trying to show off and often exagerates/creates stories. Can also be used to refer to someone who thinks they are a lot 'cooler' than they are.
"I'm really good on the guitar, my band supported the Foo Fighters" "Shut up Das Butt"
"He claims to be a fantastic cricketer, but he's a Das Butt"
"I was the coolest guy in my school" "yeah right Das Butt you used to get bullied all the time!"