What it means
Used to call dibs on the front passenger seat, or by extension to claim first pick of anything before your mates can react. You just shout shotgun the second the car is in sight and the seat is yours by playground law. Works for the last slice of pizza too, as long as everyone heard you.
Usage examples
"Shotgun! I called it before we even reached the car, so you lot can fight over the back seats while I crank the aircon."
"My little brother shotguns the front seat every single morning, then complains the moment I touch the radio."
Where it comes from
Comes from the old American stagecoach days, when the man riding up front next to the driver carried a shotgun to fend off trouble on the road. The seat next to the driver kept the name long after the gun was gone.
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