a person afflicted with psychosis
"The neighbor watched with concern as the psycho paced frantically, shouting about monsters that no one else could see."
A person who is psychotic or otherwise insane.
"She decided to take the psycho on her first semester of undergraduate studies."
A class, at a college or university, in which psychology is taught.
In plain English: A psycho is someone who acts crazy, dangerous, or completely out of control.
"Stop being such a psycho and let me finish my story."
Usage: Use the informal noun "psycho" to describe an irrational or dangerous person, but avoid it in formal writing where "psychopath" or clinical terms are preferred. Do not confuse this slang term with the academic sense of a psychology class, which is unrelated to mental illness.
Psychotic, or otherwise insane.
"After losing his job and moving to a new city, he became convinced that every stranger was plotting against him."
In plain English: Psyco means crazy or mentally unstable.
"Don't call my sister psycho just because she is very intense about her diet."
The word psycho is a clipped form of psychotic or psychopath combined with the suffix -o, which means "person with a characteristic." It follows the same pattern as the slang term sicko.