Green’s Dictionary of Slang

shotgun! excl.

[abbr. ride shotgun under ride v.]

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

[US]P. Munro Sl. U. 170: Shotgun! [...] Dibs on the front passenger seat!
OnLine Dict. of Playground Sl. [Internet] 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’.
Twitter 27 June [Internet] dunno why anyone calls ‘shotgun’ when that just means you have to be the car butler.