If you call shotgun, you get the privilege to sit in the passenger seat in front, next to the driver.

stagecoach guards rode shotgun - they just didn't call it that in the 1880s, as far as anyone has yet discovered. The term "riding shotgun" to refer to the guard sitting next to the driver doesn't emerge from the Old West but rather from movies and TV shows about the Old West. To date no one has found a cite for "riding shotgun" during the time stagecoaches were actually used.

The earliest usage we've found in pulp fiction occurs in the March 27, 1921 issue of the Washington Post's "Magazine of Fiction," in a story entitled "The Fighting Fool" by Dane Coolidge.(See Examples)

In the classic 1939 movie Stagecoach: Curly, the sheriff, says, "I'm gonna ride shotgun," and John Wayne expresses surprise at seeing him in fact riding shotgun later. So we have references from pulp fiction and from the movies (but not from the Old West itself) using the term "riding shotgun" to refer to the stagecoach guard.

Stagecoach revived interest in westerns as a movie genre; in the 1950s they became a staple of television, too. Not surprisingly, catchphrases from westerns soon found their way into everyday speech.

So when does "riding shotgun" get transferred from stagecoach to automobile? The Dictionary of Americanisms (1951) doesn't mention "riding shotgun." We're not sure whether absence of a phrase is evidence, but it's certainly indicative. The first usage in print relating to automobiles, is - ready? - 1954. Dropping "riding" and using the simple "shotgun" (as in "I call shot...