Psycho is a slang term that can mean:
1. A crazy or mentally unstable person (informal usage)
2. Short for "psychopath" - a person suffering from chronic mental disorder with abnormal or violent social behavior
3. In psychology, relating to the mind or mental processes
4. As a prefix (psycho-), meaning relating to the mind or psychology
5. In popular culture, refers to the 1960 Hitchcock film "Psycho"