Why call the front passenger seat "shotgun"?

Factual Questions
1

Every time my family is getting ready to drive somewhere, my brother and I fight for the front passenger seat. The official way of reserving the front seat is by being the first one to yell “shotgun!” As primitive as this might sound, after years of experimenting, we have found that this is as fair and commotion-free as it is going to get.

Well, the other day, after claiming the front seat in this fashion, I got into a huge argument with my dear father over the origination of this phrase. He seems to think that it originated from traditions in the Old West. In the wagon, he says, while the driver steered the horses, it was the front passenger’s role to hold the shotgun and fend off any predators, hunt prey, etc. I thought that this explanation sounded fishy.

Does anyone have any light to shed on this issue?

2

You bro is correct.

3

I always figured it came from having an assistant/guard sitting next to the stagecoach driver, carrying a shotgun to deal with any attackers. It would be natural to call his position (both his role and his seat) “shotgun.”

4

Your father has it right. Whether or not this practice was typical in the Old West itself, it has been depicted many many times in TV and movie Westerns. Watch almost any movie or TV show featuring a stagecoach.