Definition & Meaning of "[ride|sit] shotgun"

What is the origin of the idiom "ride shotgun" and when to use it?

The idiom "ride shotgun" has its origin in the American West during the 19th century. Back then, stagecoaches had a person seated beside the driver, often carrying a shotgun to provide protection against potential threats. This person, known as the "shotgun messenger" or "shotgun guard," was responsible for guarding the coach and its passengers. Over time, the term evolved into a colloquial expression used to describe someone sitting in the front passenger seat of a vehicle, typically next to the driver. It has taken on a more casual meaning, signifying the front-seat passenger position, with no specific security duties implied.