Probably arose in early-20th-century Western fiction and movies where an employee or friend armed with a rifle or shotgun would ride next to a stagecoach driver for protection from bandits or Indians.
- (idiomatic, slang) To ride in the front passenger seat of a vehicle, next to the driver.
- When both kids want to ride shotgun with Mom, they'll just have to take turns.
- (idiomatic, slang, figuratively) To accompany someone in order to assist and protect.
- He attended the meeting to ride shotgun for the sales team, in case anyone had a technical question.