Similar terms used to be used in the U.S. because in a collision the steering wheel might provide some protection to the driver, but the person next to them would go through the windshield.
The terms has gone out of use over time with the introduction of lap belts, three-point restraints, and finally air-bags.
Before safety belts and airbags, and before safety glass, in a frontal collision the driver would sometimes be caught into the steering wheel and get hurt, but the passenger would smack his face into the windshield and die (or get thrown straight through). Also when you get T-boned from the right (which is the expected result of not seeing a vehicle that has the right of way), the driver’s fine but the passenger’s screwed.
So, statistically, there was a time when the passenger in a car crash had a higher fatality rate than the driver. This isn’t true any more, but the idiom stuck.
IIRC, the much-touted-in-car-commercials-in-the-'60s ‘deep dish steering wheel’ was developed as a safety device to mitigate the driver being impaled. (Plus it looked sporty.) I think the idea was that the rim would absorb some energy so that the hub would not penetrate/cause much blunt-force damage. That, plus collapsing steering columns (which I think were mandated in 1968), no doubt saved many lives.
Another US driving tradition that dates from the stagecoach days is the practice of driving on the right. In Jolly Old England ™, people rode on the left so that they could draw a sword with their right hand and have it available to whack someone passing by on their right. In the 13 colonies and later the US, people rode with guns. If you were right-handed, it was easier to shoot a long gun (musket, shotgun, rifle, etc.) toward your left. So people kept to the right so they could draw and fire on a moment’s notice if the guy coming turned out to be a bandit or some other undesirable person.
In Spanish from Spain asiento del copiloto; I’ve heard other people call it asiento del navegador but in any case it means “seat where the person with the maps goes”. In English class (British, always British) we were taught “passenger seat”; when I first encountered “shotgun” it took me a while to figure out what were the Americans saying.
There’s no evidence that the term was used until the early 1920s. Stagecoaches drivers did have people riding shotgun. It just appears they didn’t call it that.
As Martini Enfield pointed out, I wouldn’t say this was a misunderstanding really, just another example of an evolving language. It’s a very common and well-understood phrase among those of my age group (28) and younger.
So cultures unfamiliar with the whole ‘shotgun’, as designating a specific seat in the car, have now taken it to replace, ‘dibs’. As in, ‘Dibs on the doughnut with sprinkles!’
I bags/bag/bagsy the front seat/first cake/whatever. Bags is used idiomatically, not in accordance with common grammar.
“Bagsy the front seat!!” “Bags the front seat!” (Or cake, or whatever). Or just bagsy. “Bagsy first in the loo” is a common one after a long journey home. It only counts when you’re either walking towards home or at least pulling up there though - you can’t bagsy first in the loo two miles down the road unless you’re saying it in the sense of “if you don’t let me go first, I actually am not going to be able to control my bladder/bowels, so trust me, you want me to go first.”
Then there is the curious usage by truckers, I guess (first heard it on a TV show about truckers) where the guy who is not driving is called the “swamper”.