An Etymological Dictionary of the German Language/Annotated/finster
finster, adjective, ‘dark, gloomy, morose, sullen,’ from the equivalent Middle High German vinster, Old High German finstar; Old Saxon *finistar, as an adjective, is not found, but it may be inferred from a substantive with the same sound, meaning ‘darkness’; the stem is essentially German, but a series of phonetic difficulties (see düster) hamper the discovery of the type. In Old High German there exists besides finster an Old High German dinstar, Middle High German dinster, whose initial d must have been substituted for an earlier (Old Saxon, Gothic) þ; to these Old Saxon thimm, ‘dark,’ corresponds. The interchange of þ and f, judging from the parallel forms under Feile and Fackel, cannot be denied. In that case the root would be þem (see Dämmerung). But Old Saxon thiustri, Anglo-Saxon þŷstre ‘gloomy,’ have no connection with it.