A play on the word "hardcore". It is rape porn. It goes beyond S&M because it involves actual rape and abuse. Usually humiliation, pain, hurting, etc are involved. Although its something people ask for on message boards, very little of it exists, because most people do not enjoy seeing others harmed.
"I'm an arrogant little fucker who wants to see people raped and humiliated, can you get me some hurtcore to jerk off to."
"You want to see hurtcore? I have a crow bar and a camera right here. Maybe you and I can make some for you."
"Oh, uhhhh, uhh actually nevermind."