What does "calling shotgun" have to do with reservation of a seat near the car driver?
According to the Phrase Finder, the related phrase riding shotgun has the following original meaning:
For example, the site writes that:
That is, it is a Hollywood reference to the practice of having an armed guard in a stagecoach. The site writes that while there were armed guards in the 1800s, the position was not referred to as "riding shotgun" until later:
The Oxford English Dictionary dates the phrase to 1913, which agrees with some other passages in the Phrase Finder's article:
The use was then adapted in the 1960s (according to the OED) to refer to the front seat of any car. There is no strict date on when "calling shotgun" came into use, but I suspect it came after the 1960s, once the use of shotgun referred to the front passenger seat of a car. Then, "calling shotgun" would be to "calling dibs"; one was laying claim to a particular thing.
This is a bit toungue-in-cheek, and I can't find a good reference for it. But as an Okie I think I may qualify as an expert on this.
In a pickup truck there are three different acknowledged riding positions (well, four if you count the driver).
Now this terminology is a bit dated, as pickups these days double as passenger vechicles, and thus may have bucket seats (no "bitch" position) and back seats which don't fit in this scheme. However, "Shotgun" at least is universal, and has gone into the lexicon for passenger cars and any other conveyance that sports side-by-side passenger seating.