duff

This term carries a heavy sense of inadequacy or failure, often used in a mocking or self-deprecating way. It suggests a lack of natural skill or a clumsy execution of a task, creating an image of someone who trips over their own feet or fumbles a simple operation.

In British English, it frequently describes something that is broken or non-functional, such as a duff battery. This shifts the feeling from personal incompetence to mechanical failure, implying the object is useless or a "lemon."