What do you mean, someone with a loose screw?

It means a person with abnormal brain. According to relevant public information, loose screw means that the screw in a person's head is derailed, which means that the screw is loose, the head can't work normally, and the head is abnormal, which means swearing (or laughing at others). Screw is a tool to tighten instruments and parts step by step by using the physical and mathematical principles of circular rotation and friction of the inclined plane of an object.