An Etymological Dictionary of the German Language/Annotated/Schwefel
Schwefel, masculine, ‘brimstone, sulphur,’ from the equivalent Middle High German swëvel, swëbel, Old High German swëval, swëbal, masculine; the f of the Modern High German form can only be explained by the influence of Low German, as is shown by the double forms in Middle High German and Old High German. A common Teutonic word; compare Dutch zwavel, Anglo-Saxon swefl, Swedish swafvel, Gothic swibls, ‘sulphur.’ Latin sulpur (for *suplur?) is probably not allied. If the Old Teutonic swebloz, ‘sulphur,’ is a primitive loan-word, it may perhaps be connected with the old Aryan root swep, ‘to sleep’ (Sanscrit svápnas, Latin somnus, Greek ὕπνος, Anglo-Saxon swëfn); compare Anglo-Saxon swębban, ‘to kill,’ Old Icelandic svœ́fa, ‘to kill, lull to sleep’; Schwefel may then be literally ‘stifling, killing, soporific stuff.’