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 Indians.
- (idiomatic, slang) To ride in the front passenger seat of a vehicle, next to the driver. [from 1963]
- 2018 December 25, Austin Murphy, “I Used to Write for Sports Illustrated. Now I Deliver Packages for Amazon.”, in The Atlantic:
- Before sending me out alone, the company assigned me two “ride-alongs” with its top driver, the legendary Marco, who went out with 280 packages the second day I rode shotgun with him, took his full lunch break, did not roll through a single stop sign, and was finished by sundown.
- 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.