1. adjective, offensive slang A shortening of "psychopathic," used generally to mean crazy or mentally unstable and likely to engage in harmful antisocial behavior. I still get weird phone calls from my psycho ex-boyfriend from time to time. I had a real psycho professor last year who used to scream and shout at us throughout the lecture.

2. noun, offensive slang A shortening of "psychopath," used generally to mean a crazy or mentally unstable person, typically one who engages in harmful antisocial behavior. Don't be such a psycho—if you don't want something, just say so, don't throw it across the room at me! You've got to be careful at night, man—a lot of psychos out there.