The idiom "have a screw loose" means that someone is behaving in a strange or irrational way, or that they are mentally unstable. It is often used to describe someone who is acting in a way that is not normal or expected.
For example, if someone is talking to themselves, laughing uncontrollably, or doing something that is dangerous or bizarre, you might say that they "have a screw loose."
This idiom comes from the idea that if a screw is loose in a machine, it can cause the machine to malfunction or behave in unexpected ways. Similarly, if someone "has a screw loose," it means that there is something not quite right with their thinking or behavior.
Overall, this is a slang expression that is used in informal situations, and it is not considered polite or respectful to use it to describe someone who is actually suffering from a mental illness.
• My car is making a weird noise, I think something is wrong with the engine, maybe I have a screw loose.
• The politician's plan to cut taxes without reducing spending shows that he has a screw loose.
• She always talks to herself and laughs for no reason, she must have a screw loose.
• The scientist's theory about time travel is so outrageous that many people think he must have a screw loose.
• He always forgets simple things, I think he has a screw loose.
• She thinks the moon is made of cheese, she must have a screw loose.