Originated from the Steam clan The PSF, "Petrol Station Flowers", and their crazy antics.
Yeah, classic PSF.
It is generally used when addressing a close friend as a term of endearment, in much the same fashion blacks stereotypically use racial slurs to express their friendship. Another similarity to the racial slurs parallel is that it is NOT acceptable to be outside a friendship circle and refer to someone within said circle as a "PSF."
The term "PSF" is often lengthened to its root "piece of shit faggot," depending upon the user and the situation at hand. When pressed for time or around someone not within the aforementioned circle, PSF becomes one's first choice. When not rushed and not bothered, the elongated term is the way to go.
Person 2: "I know. Fuckin' PSF. Give him shit when he gets here."
This is usually a self-given title.