The act of riding in the bed of a pickup truck due to lack of available seats inside the cab. Favored by rednecks, frat boys, and illegal immigrants.
"There's no room up front, looks like you guys are riding mexican tonight"
Riding in the trunk of a car. Usually happens when the car with not enough seats for all your friends.
"Wait, I thought there were 7 of you, Joe's car only holds 5."
"Yeah, Steve and I were riding Mexican."