Back in the day (the western days) people were hired to ride shotgun to protect the driver of the stagecoach, most often the coaches carrying money and valuables. ( source ) The term has morphed into riding in the front passenger seat of a car. And as in all competitions there are rules for calling and riding shotgun as outlined here .
"We realized that the important thing was not the film itself but that which the film provoked." Fernando Solanas