A screw loose
Meaning
To have a screw loose means someone is slightly eccentric, irrational, or mentally unsound.
Origin
Imagine the clatter and whir of 19th-century industrial machinery. Every cog, every piston, every beam needed to be fastened tight for the machine to operate smoothly, efficiently. But let just one vital screw work its way loose, and suddenly the whole contraption would begin to falter, to wobble, to behave erratically—perhaps even dangerously. This tangible image of mechanical failure, where a small flaw causes significant disruption, vividly leaped from the workshop floor into the human sphere. If a person was acting strangely, unpredictably, or with an obvious lack of sound judgment, they were said to "have a screw loose," as if their internal gears weren't quite meshing, their mental faculties a little bit undone.
Examples
- After he started talking to himself in the supermarket, I really began to think he had a screw loose.
- Don't worry about his strange ideas; he just has a screw loose, but he's harmless.