Green’s Dictionary of Slang

perverted, insane.

1976		P. Conroy Great Santini (1977) 265: That’s a little sicko, Popsy, a little sicko-sexual weirdness.
1979		L. Bangs in Psychotic Reactions (1988) 294: I was going to [...] tell her she was a sicko weirdo Goodbar so-’n’-so.
1980		A. Maupin More Tales of the City (1984) 57: This wasn’t some sort of sicko fantasy trip.
1991		P. Cornwell Body of Evidence (1992) 376: He was reciting what he knew from conversations with his sicko pal.
1998		D. Hecht Skull Session 269: The place was so badly vandalized [...] the damage level was sicko.
2001		N. Griffiths Sheepshagger 1: What was yewer verdict, Griff? ‘Fuckin sicko pervert’, was it?
2021		J. Ellroy Widespread Panic 3: Caustic cads that capitalized on a sicko system.