We have the same word in French (with a spelling variation, but that is to be expected), so this word likely comes from a foreign language, and hence isn't present in all English dictionaries, or something along these lines. And French has probably borrowed it from somewhere else as well (from what I can tell, "fenestra" is the latin for "window").
But this wasn't quite exactly a thread for etymology, and so I will stop digressing. I shall now leave the literati to debate about these strange words.
(Literati has a nice ring to it, but I cannot think of something better than this one to bring to the thread)