A 6PBF is an acronym for 6 Pack of Back Flaps. When an obese woman is wearing a tight shirt and her back flaps show through, she exposes her 6PBF. The 6PBF consists of the first two flaps on top of the bra, the main two rolls underneath the bra, and of course the muffin top above the pants. Also known as the Half Rack of Back Flaps(HRBF).
I was going to have sex with her, but I was turned off by her 6PBF. Then I got drunk and did it anyway.