have a screw loose

Example: “Anyone who swims in that river in March has a screw loose.”

Origin: Draws on the plain mechanical reality that a single loose screw, easy to overlook, can throw off an entire device's performance or cause it to malfunction unpredictably — a small, hidden flaw with outsized consequences. Some sources trace the image to the late-1700s cotton industry, where newly mechanized textile machinery that broke down or produced flawed goods was informally said to have "a screw loose" somewhere inside it. The specifically human, mental application is documented by 1810, in an Edinburgh Review article describing an oddly behaving character exactly this way, and the phrase caught on widely in Victorian England — a period notably fascinated with using mechanical analogies to describe human character and behavior, treating an eccentric or unstable mind as a finely made machine with one small part gone wrong.