also known as "PF", the condition where a male's penis- whether donning underwear or not- is unknowingly, or knowingly showing through his pants and consistently hitting said pants as he walks, runs or generally moves. Females tend to whisper "PF" to tell one another that they've spotted penis flop on a male.
"Check it out, girl! PF! PF! Anfernee's got penis flop! I can totally see it through his track pants. It's all flappin against his pants and shit!"