term is derived from the observation that Mexicans will often all sit in the front with each other, leaving the back seat clearly vacant, probably so one person is not left to be a loner in the back seat.
Nothing against Mexicans, purely observational. I'm Salvadorean, and probably have more Mexican friends than Salvadoreans.
Driver: Ready to go? We gotta pick up Joey, Tom, Kelly and Sarah too.
Friend: Doesn't your car only fit four?
Driver: Yah... We're going Mexican, no worries.