An expression I noticed recently was “Shotgun!“, as used as an exclamation to claim the front passenger seat of a car. According to the Urban Dictionary, back in the days of the wild west in America when people travelled by stagecoach, the seat next to the driver was usually occupied by a man with a shotgun who was there to protect the travellers and their valuables. He was said to be ‘riding shotgun’, the word shotgun became associated with the seat next to the driver.
According to The Straight Dope, the term ‘riding shotgun’ was first used in stories about the wild west rather than in the wild west itself – the term ‘shotgun guard’ was used though. The earliest usage of the expression in relation to stage coaches appears in an issue of the Washington Post’s “Magazine of Fiction” from 27th March 27 1921 in a story called “The Fighting Fool” by Dane Coolidge, and it was first used to refer to the front passenger seat in cars in the 1950s.
Shotgun is used mainly in American English, as far as I’m aware. If I wanted to lay claim in a similar way I might say “bagsy the front seat”. Bagsy (/ˈbagzi/) can also be used to unofficially reserve many other things for oneself, e.g. “Bagsy the sofa/chair/remote control/etc.” Variations on bagsy include bags, bagsie, begsie, bugsy and dibs, and I’m sure you’ll tell me about others. You might also say things like, “I’ll bag the best seats”.
The origins of bagsy and dibs are uncertain, but a number of theories are discussed on Wikipedia.