When something is so disgusting that you gain sick pleasure from it or get turned on because it's just so gross.
Like when the perky cheerleader in the horror film gets her head chopped in half and blood and gore squirts everywhere, and you feel that rush-type-thing as you scream. Or when you're reading a really terrifying story and you're like squirming around in anticipation of what happens next and all giddy and scared.
Basically getting excited by something creepy.
"This is so nasty it's awesome! I'm totally going to have a horrorgasm if the vampire tears out that dude's jugular!"
The state of being horny, for people who are attracted to killers from slasher films, monster stories, cryptids, lovecraftian creatures and such. Such a state also applies to anyone in heat who identifies as a monster fucker.
"I got so horrorny while watching the Halloween films on Discord last night. I had to leave the call. I can't help it, seeing Michael Myers in action gets me completely soaked."