Psycho is slang for a person who is psychotic or psychopathic. http://en.wikipedia.org/wiki/Psycho and psycho ( exist in the Oxford dictionary ) psy·cho noun(pl. -os) (informal) a person who is mentally ill and who behaves in a very strange violent way > psy·cho adjective Source : Oxford Advanced Learner's Dictionary

"...which is not written in oxford dictionary" Maybe because this word doesn't exist in English????

cuz u spelled it wrong :P

If you call someone "a psycho" it means that they are acting strangely or violently. They might be affected by medication, alcohol, drugs or have a mental illness.