Stands for Potential Fat Factor. It describes a person (usually female) that although not technically fat, it appears as if it has the potential or tendency to become such. A good indicator for PFF is usually given by observing the subject's mother.
"Kate is cute, but looking at her puffy cheeks and round ass, I get the impression she has some PFF"
"Dude, why did you break up with Sophie?"
"I met her mother and she is HUGE! Turns out Sophie has a lot of PFF"