The History
The history of calling "Shotgun" goes back to the days of covered wagons and the Wild West. On a trip across the plains, the driver of a wagon would hold the reins of his horse team and concentrate on driving. This left him and the occupants of his wagon susceptible to sneak attacks from bandits and thieves. To avoid this atrocious circumstance it became necessary for one person to sit next to the driver with a shotgun and fend off the enemy.
Defending against bandits is no longer the priority of Shotgun however, but it has evolved into a pre driving ritual that is experienced before almost every car ride across America and even the world. Because of the obvious evolution that has already occurred with Shotgun, we ask you to consider Shotgun as a living entity and be aware that it is always changing for the better good of society.
Source(s):
http://www.shotgunrules.com/