Shotgun is the best seat in a car because you have a dominant position and the best view and more leg room. However, if the driver is short and has the seat pulled forward, shot bum can have more leg room than shotgun or cobain, who will be ready to blow his brains out because he has no leg room.
Skippy - "Dude, I called it like 3 minutes ago."
Bif - Okay, I want shot bum then because Kelly has the seat all the way forward.
Matt - I guess that means I get cobain