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:
To travel as an armed guard next to a vehicle's driver. Latterly, (chiefly in the USA) - to travel in a car's front passenger seat.
For example, the site writes that:
The reference is to the US stagecoaches that were an essential feature of Hollywood westerns - usually being chased by Indians or bad guys in black hats. In the 1939 classic film Stagecoach, George Bancroft plays Marshal Curly Wilcox who is featured riding shotgun in screens throughout the film, to protect the coach from the pesky Apaches. He mentions the term explicitly in the dialogue:
"You boys take care of the office for a couple of days. I'm going to Lordsburg with Buck. I'm gonna ride shotgun."
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:
It seems quite plausible that the term riding shotgun would have been used, but it appears that it wasn't - not until well after stagecoaches had gone out of use and people started making westerns. Although we have 20th century references to people riding on stagecoaches with shotguns from films and newspapers, there are no accounts from the 19th century that call this riding shotgun.
The Oxford English Dictionary dates the phrase to 1913, which agrees with some other passages in the Phrase Finder's article:
1913 A. H. Lewis Faro Nell iv. 105 If thar's money aboard, an' the express outfit wants it defended, they slams on some sport to ride shotgun that trip.
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).
- Shotgun - passenger seat. As mentioned by @simchona, taken from westerns. Due to the availablity of the passenger-side window, this is the only seating position from which a weapon could easily be used.
- Bitch (somewhat profane) - The middle of the bench seat. So named because this is where the pickup driver's girfriend tends to sit, even when "shotgun" is empty.
- Dog - In the bed of the pickup. So named because this is where dogs tend to ride.
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.