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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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
n. a psychopathic person; a crazy person. Pat is turning into a real psycho.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.