An Etymological Dictionary of the German Language/Annotated/Biest
Biest, masculine, in Biestmilch, from the equivalent Middle High German biest, Old High German biost, masculine; compare Anglo-Saxon beóst, and its derivative Anglo-Saxon bŷsting, English beastings, biestings. Modern High German dialects have also remarkable parallel forms with br, like Old Icelandic á-brystur, ‘beastings,’ e.g. Swiss briešt (brieš), which may be connected with Brust, Old High German brust, Anglo-Saxon breóst. Beyond the Teutonic group (whence Old French bet, Modern French béton is borrowed) the stem has not yet been traced; it is most frequently compared with the equivalent Greek πῦός, Sanscrit pîyûša. Yet a Teutonic root bius seems to underlie biese, beise, ‘to milk,’ in the Wetterau dialectic.