Schooly D is known as one of the first people to make rap music that could be classified as gangsta rap. Another name that comes to mind is the name of Just-Ice who also fits the early gangsta bill. The song PSK, What Does That Mean? is an out and out hip hop classic and should be known anyone that claims to love the music.
So PSK stands for The Parkside Killers which was a real outfit that ran in Philly based out of a neighborhood of the same name. Just think of the landscape of rap music if Schooly D didn't help set off the genre that groups like NWA took the baton and ran with.