I have no idea, actually. I just see it used other places online, and from context, I'm guessing it's something related to kinks?

Sunday isn't an outright psycho though... He doesn't even really have that much of a sob story (especially compared to others). He's just...angsty, which I guess is a type that people like? And he's preachy...and, I guess some people like his philosophy? *shrug* iono