The guy you sit next to in class who's killed so many brain cells with drugs that he has to ask "Dude, where am I?" every five minutes. A burnout is a stoner who didn't stop until every last brain cell was fried, and even though every brain cell is now fried, is still smoking at least five blunts a day.
Burnout: "Dude, where am I and why is everyone being so quiet?"
normal person:"This is your brother's funeral"
burnout:"Ah man, that sucks. Wait, I had a brother?"
burnout:"Woah, that's crazy! Wait-did you know that when I hit myself in the head it sounds like it's hollow?"
normal person: "Why am I not surprised?"