1
general::
Guard someone or something while in transit, as in The reporter found himself in the odd position of riding shotgun for an accused mobster. This term alludes to the armed defender of a stagecoach who sat beside the driver to protect against marauders and bandits. Later it was transferred to anyone riding in the front passenger seat of a motor vehicle, as well as to the more general function of protection. [Mid-1900s]
American Heritage Idioms