Calling shotgun is an American idiom used to express your desire for the front row car seat, or claim it, when a group of people go out together. Especially if a family has more than one young kid, the siblings may compete to sit in front with the driver. So, they usually run to car and say "I called the shotgun"! This saying has roots to Wild West era, 160 years ago, when the person next to the driver was protecting him:
https://www.businessinsider.com/wild-west-origin-shotgun-front-seat-car-2019-8
