Green’s Dictionary of Slang

1. (orig. US, also shotgun seat) the seat next the driver in a vehicle, thus call shotgun, to claim the seat next to the driver.

1952		Arizona Dly Star (Tucson, AZ) 25 Feb. 9: [photo caption] M.M. Garrett getting ready to ride the shotgun seat next to driver, Frank Clark.
1960		Des Moines Register (IA) 27 July 4/3: The cowboy-hatted Johnson [...] rode in the shotgun seat.
1969		Pittsburgh Post Gaz. (PA) 24 Oct. 29/1: I’ve ridden many a wide-eyed mile in the ‘shotgun seat’.
1978		R. De Christoforo Grease 42: We piled into Cousin Finn’s Chevy [...] fighting over the shotgun seat.
1988		Eble Campus Sl. Mar. 9: shotgun – the front seat of the car, the privilege of sitting in which goes to the first who says ‘shotgun’ when walking with a group towards the car.
1992		R. Price Clockers 401: Strike looked down at the brown bag on the shotgun seat.
1997		Simon & Burns Corner (1998) 317: One the way home, Will’s brother is back at shotgun, with Gary riding alone in the empty pickup bed.
2000		G. Pelecanos Shame the Devil 79: From the shotgun bucket, Anna Wang side-glanced Stefanos.
2010		Codella and Bennett Alphaville (2011) 275: I drove, with Angelo in the shotgun seat.
2013		S. Barber ‘Killer, Duck and The Boys’ in ThugLit Apr. [ebook] In the car he called shotgun.
2015		P. Beatty Sellout (2016) 22: The gangbanger cruising up and down [...] and sitting ‘shotgun’ because he has one.
2020		(con. 1991-94) W. Boyle City of Margins 26: She’s on the Belt Parkway [...] a bag from Macy’s riding shotgun.

2. (drugs, also shotgun rider) a guard who accompanies a drug courier to ensure that all goes well on their trip (and that they do not attempt to abscond with the consignment).

1978		R. Daley Prince of the City 56: [They] arrested three men: Andretta, Andretta’s shotgun rider, and Joe, whose name was Joseph Marchese.
1998		Source Oct. 158: ‘Shotguns’ [...] who escorted couriers from New York to Nigeria and back to ensure the heroin’s safe delivery.

3. any form of guard.

[1909		W.M. Raine Brand Blotters (1912) 61: The Mexican driver was half asleep and the ‘shotgun messenger’ was indolently rolling a cigarette, his sawed-off gun between his knees].
1988		D. Woodrell Muscle for the Wing 48: I heard he stood shotgun at crap games and such.
2002		J. Cameron Hell on Hoe Street 99: Outside she got a cowboy waiting in his wagon and shotgun leaning out the other window.