A girl you really want to fuck or have already fucked and are commenting on. This usually orientates itself around the condition of her booty but can also reference the relative tightness of her cooch as well. In rare cases the phrase can refer to her rack along with her booty and cooch as a statement on the entire package in bed.
Alternative definition: Noun: A really small car.
Yesterday I fucked this girl and she was such a tight ride I almost couldn't get it in.