At Language Log there's a post about how English-speakers use "open" and "closed", which are not grammatically the same sort of thing, in opposition to each other -- "open"/"close" or "opened"/"closed" would, on the surface, make more sense. (Compare French ouvert and fermé, which are both past participles.)
I won't try to summarize the linguistic content of the post; I'm not a linguist, although I did go through a phase where that seemed interesting.
But in mathematics-land, open and closed aren't even opposites, in the sense that open means not-closed and closed means not-open. Of course the complement of an open set is closed, and vice versa, but that's a more complicated relationship, because now we're talking about two sets, not one. This is one of about a zillion examples of how we take perfectly good natural-language words and give them specific meanings (group, ring, field, set, class, ...), which may or may not be preferable to making up entirely new words as some other fields (biology comes to mind) prefer.