The expression “riding shotgun” is derived from the days of stagecoach travel when a special armed employee of stage would sit beside the driver, carrying a shotgun to provide an armed response in case of a threat to the cargo, which was usually a strongbox.
Absence of an armed person in that position often signaled that the stage was not carrying a strongbox, but only passengers. However, apparently the phrase “riding shotgun” was not coined until 1919. It was later used in print and especially film depictions of stagecoaches and wagons in the Old West in danger of being robbed or attacked by bandits.