crazy; insane; out of one's mind
Colloquial expression meaning someone is mentally unstable or acting irrationally. Often used in casual conversation.
Saying something like that, he must be crazy.
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'.