expression, i-adjective
Colloquial expression meaning someone is mentally unstable or their thinking is irrational. Often used in casual, sometimes derogatory, contexts. Compare with the more explicit 頭がおかしい.
See also: 頭がおかしい (あたまがおかしい)
あいつ、頭おかしいんじゃない？
That guy is crazy, isn't he?
こんな時間に電話してくるなんて、頭おかしいよ。
Calling at this hour is nuts.