An Etymological Dictionary of the German Language/Annotated/Welle
Welle, feminine, ‘wave, billow, swell,’ from the equivalent Middle High German wëlle, Old High German wëlla, feminine; a word peculiar to High German, but found, however, in the Old Teutonic vocabulary. It is primitively allied to the equivalent Old Slovenian vlŭna, Lithuanian wilnìs, and is based like these on an Aryan root wel, ‘to turn, roll,’ which appears in Old High German wëllan and wallen. Compare Latin volvere, Greek εἴλύειν, ‘to roll,’ as well as Sanscrit ûrmi, equivalent to Old High German walm, Anglo-Saxon wylm, ‘billow.’