I think it comes from the days of coaches when someone would ride beside the driver with a gun for protection.

@wilma actually, that’s partially incorrect. Men armed with shotguns rode in the “passenger seat” of coaches in the West, but this position wasn’t called “riding shotgun” at the time. The phrase “riding shotgun” actually emerged later from movies about the armed people who “rode shotgun.”

Source

@Fiddle_Playing_Creole_Bastard so the term was made up later, but at the time of people actually riding shotgun, they never called it that, correct?

Actually, there’s a good description of this from the opening pages of Dickens’ A Tale of Two Cities as a lone rider attempts to hail and stop a coach to deliver a message on a dark night on a lonely road. He nearly gets his head blown off in the attempt. That image, which I assume to be more or less accurate (depending on the firearms that a “shotgun” rider might have had in the latter part of the 18th Century) predates any “stagecoach” riding in the American west (even if the writing itself doesn’t).

@wilma you got it.

I thought it came from drive by shootings.

@ragingloli nope.

http://en.wikipedia.org/wiki/Riding_shotgun

Well it would certainly fit.

I like that they actually did sometimes call the armed rider a “shotgun messenger”. Drawing from the comment that @CWOTUS contributed about A Tale of Two Cities it sounds like that rider was well equipped to deliver a message in no uncertain terms.

@ragingloli Classically, the driveby is performed with hand- or submachine guns.