Claiming ownership of the right to "not" do something. There is no way to beat shotgun not, it is final and absolute in it's power.
If the doorbell rings, the last person to call shotgun not must answer it.
Used to "shotgun" not doing something. Said shortly, and usually quickly, after someone asks for a task to be completed. The last person to say "Shotgun not" must then complete the task.
fathead: I got just got an oz, lets go for a session!
azz: Fuck yeah!
pedro: Got a billy?
fathead: Nah, we need to get one aye,
pedro: shotgun not
fathead: shotgun not
azz: shotgun not
spook: shotgun not...fuck it
fathead: hehe get goin spook, don't come back till you've got one