The expression "riding shotgun" is derived from "shotgun messenger", a colloquial term for "express messenger", when stagecoach travel was popular during the American Wild West and the Colonial period in Australia. The person rode alongside the driver. The first known use of the phrase "riding shotgun" was in the 1905 novel The Sunset Trail by Alfred Henry Lewis. A shotgun messenger was a private "express messenger" and guard, especially on a stagecoach but also on a train, in charge of overseeing and guarding a valuable private shipment, such as particularly the contents of a strongbox (on a stagecoach) or safe (on a train). Wells Fargo Co. express messengers typically carried a short (or sawn-off) 12- or 10-gauge double-barrelled shotgun, loaded with buckshot. This was a most effective weapon in use against pursuing riders. Such weapons were sometimes referred to as "messenger shotguns" or, more commonly, "coach guns" (a name still used today).

Source: Wikipedia