What does "Shot Gun" mean?

During bank robberies, the getaway car would be parked outside with the driver already in the seat. The person performing the stickup (or robbery) in the bank would run out and get into the front penger seat with his firearm (sometimes a shotgun) still in his hands. The front penger robber would use his gun to attack police during a chase. The front penger seat became known as 'shotgun' and anyone that says 'shotgun!' claims their own right to sit there. The term transferred to other, non automotive purposes eventually.