Noun

1. A person who is psychotic or otherwise insane.

   The loony bin up on the hill is full of psychos.

2. A person who acts in a bizarre or dangerous manner.

   She complained that he was a psycho for driving at such a high speed in heavy traffic.

Synonyms

• (pejorative for a pyschotic or otherwise insane person): freak, loony, nutcase, wacko
• (pejorative for a person acting in a bizarre or dangerous manner): creep, freak, loose cannon, weirdo