Backpack style
When you fuck a girl from behind who is taller than you so you have to sort of climb on her back like a backpack to fuck her. It's similar to doggy style
Yo, I totally hooked up with that volleyball chick, but she was taller than me so I had to fuck her backpack style
backpack style
When your sexual partner is giving you a piggy back ride and you are giving it to him/her in the butt.
Hey Shelly why is your back sore o me and my boy friend were just backpack style all weekend.