This is the 6326th most frequent Japanese word.
サイコ
Psycho.
Here, 'サイコ' is part of the word 'サイコパス' (psychopath), referring to the psychological condition.
彼はサイコパスのような振る舞いをすることがある。
He sometimes behaves like a psychopath.
In this sentence, 'サイコ' is used as an adjective to describe something as psychotic or psychologically unsettling, often used metaphorically.
このホラー映画は本当にサイコな内容だ。
This horror movie has truly psychotic content.
Here, 'サイコ' is part of the word 'サイコロジー' (psychology), which discusses the field of psychological study.