Similar to "raining on my parade", "piss on my fuck" is a phrase intended to communicate dismay to another for a conceited act that infringes upon one's progress or character.
Ted: So I had an 80 point round in BattleField 2 last night.
Ralph: Oh, that's nothing, I had a 120 point round last week.
Ted: Dude, don't piss on my fuck. Eighty points was a good round for me.