dull tool
Definitions
A cutting or shaping implement that has lost its sharp edge and is ineffective for its intended purpose.
A figurative term for a person who is perceived as unintelligent, slow-witted, or lacking in mental sharpness.
Examples
Trying to carve wood with a dull tool is not only frustrating but also dangerous.
He realized that a dull tool requires much more physical effort to achieve the same result as a sharp one.
The carpenter spent the entire morning sharpening every dull tool in his workshop.
Synonyms
blunt instrument unsharpened tool worn edge ineffective implement useless tool