An Etymological Dictionary of the German Language/Annotated/rein
rein, adjective ‘pure, clean, downright,’ from the equivalent Middle High German reine, Old High German reini, older hreini, adjective; corresponding to Gothic hrains, Old Icelandic hreinn, Old Saxon hrêni, North Frisian rian, ‘pure’; in Dutch and English, correspondences are wanting. The Modern High German sense (for which in Bavarian and Swiss sauber is mostly used) is not found in the dialects; e.g., in Rhenish-Franconian and Swiss it signifies only ‘fine ground, sifted’ (of flour, sand, &c.), and belongs therefore to the Teutonic root hrī̆, pre-Teutonic krī̆, krei, ‘to winnow, sift,’ whence Old High German rîtara (see Reiter), Latin cri-brum, Greek κρί-νειν (for the adjective suffix -ni- see klein and schön). Hence ‘sifted’ may be assumed as the original sense of rein; compare Old Low German hrêncurni, ‘wheat.’