Green’s Dictionary of Slang

(US teen) when choosing seats in a car, ‘I want to ride in the front seat next to the driver!’.

1989		P. Munro Sl. U. 170: Shotgun! [...] Dibs on the front passenger seat!
2001		OnLine Dict. of Playground Sl. 🌐 shotgun n. Shouting this word is laying claim to the best seat in the car (i.e. in front next to the driver). Similar to ‘dibs’.
2016		Twitter 27 June 🌐 dunno why anyone calls ‘shotgun’ when that just means you have to be the car butler.