The phrase "riding shotgun" refers to sitting in the front passenger seat of a vehicle. Its origin dates back to the 19th-century American Wild West, specifically to stagecoach travel. In the Wild West, stagecoaches were a common means of transportation for people and valuable goods. 

These stagecoaches often traveled through dangerous territories where the risk of being ambushed by bandits or hostile groups was high. To protect against such threats, a guard or security person would sit next to the driver, carrying a shotgun to defend the stagecoach. This position was referred to as "riding shotgun." 

The term "riding shotgun" was first recorded in print in the early 20th century. An example is from the 1905 book "The Sunset Trail" by Alfred Henry Lewis, which mentions, "They saddled and rode shotgun with the drivers." The phrase became more widely known and popularized in the mid-20th century, especially through Western movies and television shows.