A "loose screw" refers to a fastener that is not tightly secured in place, which can lead to instability or malfunction in various objects. This term is commonly used in mechanical contexts, such as in machinery, furniture, or vehicles, where screws hold components together. If a screw is loose, it may cause parts to shift or become misaligned, potentially leading to damage or failure. In a figurative sense, "loose screw" can also describe someone who is acting irrationally or unpredictably. This expression suggests that a person may not be functioning at their best, similar to how a loose screw can disrupt the proper operation of a device.