An Etymological Dictionary of the German Language/Biest
Biest, m., in Biestmilch, from the equiv. MidHG. biest, OHG. biost, m.; comp. AS. beóst, and its deriv. AS. bŷsting, E. beastings, biestings. ModHG. dialects have also remarkable parallel forms with br, like OIc. á-brystur, ‘beastings,’ e.g. Swiss briešt (brieš), which may be connected with Brust, OHG. brust, AS. breóst. Beyond the Teut. group (whence OFr. bet, ModFr. béton is borrowed) the stem has not yet been traced; it is most frequently compared with the equiv. Gr. πῦός, Sans. pîyûša. Yet a Teut. root bius seems to underlie biese, beise, ‘to milk,’ in the Wetterau dial.