This might be an overstatement. There are all kinds of words which are defined not by strict taxonomic characteristics but by what are known as “family resemblance categories.” That is, the terms embrace large groups of individuals but are defined in our minds by a prototype of that group.

For example, if I asked you to come up with a sentence that had the word bird in it (I’ll give you a couple of seconds to do this) likely you’d come up with a sentence like The bird was chirping in the tree. This sentence works well if you substitute robin or sparrow, but doesn’t work at all with chicken, penguin, or ostrich. Why not? Are they not birds? Of course they are — its just that the scientific definition (that we all accept) doesn’t line up with the prototype-based definition we all have in our heads. In our heads, some birds are more bird-like than others. Same with fish.

It doesn’t mean that these words are meaningless —it means that what we think of as “meaning” is a lot more fluid than we realize.