تعريف ومعنى "neither fish nor fowl"

What is the origin of the idiom "neither fish nor fowl" and when to use it?

The idiom "neither fish nor fowl" has origins in Old English and Middle English literature, but its specific source is not well-documented. It is used to describe something or someone that does not fit into a specific category, is difficult to categorize, or lacks clear characteristics of any particular type.