rocks in one's head

Phrase

An inability to think clearly and rationally.

Japanese Meaning

頭の中に岩があるかのように、理性的かつ明晰な思考ができない状態 / 考えがまとまらず、合理的な判断が困難な状態

Anyone who'd invest in that scheme must have rocks in one's head.

あんな計画に投資する人は、正常に考えられないに違いない。

Sense（1）

An inability to think clearly and rationally.