Idiomatic phrase dating from Middle Ages meaning not one type of thing or another, defying clear categorization. The full phrase is neither fish nor fowl, nor good red herring; alternately neither fish nor flesh, nor good red herring.
This phrase originates from medieval class distinctions as apparent from diet: the clergy ate fresh fish and the common folk ate either flesh or fowl, while the poor made do with dried smoked (red) herring. The phrase, to my ear, conveys a sense of frustration at the amorphous characteristics of the thing described.