サイコ

noun

psycho; psychopath; crazy person

Casual term for someone who is mentally unstable, dangerously irrational, or extremely eccentric. Often used loosely as slang.

• あいつ、急きゅうに怒おこり出だして、本ほん当とうにサイコだよ。

  That guy suddenly got angry—he's a real psycho.

• ホラー映えい画がのサイコな殺さつ人じん鬼きが怖こわかった。

  The psycho killer in the horror movie was scary.