You've got a screw loose

Meaning

A way of suggesting that someone is acting strangely or is not mentally functioning at full capacity. It implies that a component of their brain is rattling around like a loose screw in a machine.

Blunt, teasing

Used in a sentence

• You think you can jump off that roof into the pool? You have got a screw loose.
• I think the repairman has a screw loose because he just told me my toaster needs a software update.
• My neighbor believes aliens live in his garden so he clearly has a screw loose.