crazy; insane; out of one's mind
Colloquial expression meaning someone is mentally unstable or acting irrationally. Often used in casual conversation.
あんなことを言うなんて、彼は頭が変だ。
Saying something like that, he must be crazy.
徹夜が続くと頭が変になりそう。
After staying up all night for days, I feel like I'm losing my mind.
Compound of 頭 (atama, 'head') + が (ga, subject marker) + 変 (hen, 'strange, unusual'). Literally 'head is strange', used idiomatically to mean 'crazy'.