have a loose screw

To be or seem particularly silly, eccentric, crazy, or mentally unstable. (Also written as "have a screw loose.") That guy on the corner must have a loose screw or something because he's been standing out there yelling obscenities at passersby all morning. My old Uncle Pete has a few loose screws, but he's a really nice guy. What the hell's the matter with you? Do you have a screw loose, or what?