not (quite) right in the head
Mentally unwell or unstable. The term can be used both literally and hyperbolically. Tim has been behaving very erratically lately—I'm afraid that accident has left him not quite right in the head. Liz isn't right in the head if she thinks that plan will work!
not right in the head
Mentally unsound, as in Physically, she's quite healthy for ninety, but we suspect she's not right in the head . This usage was first recorded as right in his wits in 1662.
not right in the head
SPOKEN, OFFENSIVEIf someone is not right in the head, they are mentally ill. According to Great-aunt Louise, the grandmother wasn't quite right in the head.