When three people sit in the back seat of a car, and a fourth lays across all three of their laps. The person laying is the Mexican Seatbelt, and calls "Mexican Seatbelt" upon assuming this position.
"Dude, your elbow is crushing my balls"
A Mexican seatbelt is a rope that keeps someone secure while riding in the back of a truck.
Passenger: I'm surprised Justin didn't fly out of the bed on that last speed bump.
Driver: It's cool, he's got a Mexican seatbelt.