When you fuck someone from behind while simultaneously eating pizza off of their back. Typically pizzabacking involves pizza in a box, but fresh-hot pizza can be applied directly to a recipiant's back if they need to be taught a lesson. To simulate an acid trip, try pizzabacking while watching Hanna Barbera cartoons. If you don't have a sex partner or pizza available and you still want to simulate an acid trip, try watching Hanna Barbera cartoons.
Middle Manager with Nothing to Lose: Linda do you know the difference between a Caesar Salad and a Pizzaback?
Confused Secretary: No?
MMWNTL: Good, lets do lunch. And by lunch I mean you from behind while I eat pizza off of your back.