An Etymological Dictionary of the German Language/finster
finster, adj., ‘dark, gloomy, morose, sullen,’ from the equiv. MidHG. vinster, OHG. finstar; OSax. *finistar, as an adj., is not found, but it may be inferred from a subst. with the same sound, meaning ‘darkness’; the stem is essentially Ger., but a series of phonetic difficulties (see düster) hamper the discovery of the type. In OHG. there exists besides finster an OHG. dinstar, MidHG. dinster, whose initial d must have been substituted for an earlier (OSax., Goth.) þ; to these OSax. 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 OSax. thiustri, AS. þŷstre ‘gloomy,’ have no connection with it.