1. one of the two main characters from the MTV show "Beavis and Butthead"
2. used in slang to respond to a person who has just spoken. It sometimes follows something simple which was said or denotes a slight disbelief in what was said. It could also indicate that someone isn't too bright.
"I'm gonna get rich by doing what that guy did."
"Watch, I'll fix this myself. It isn't dangerous."