An Etymological Dictionary of the German Language/Annotated/Rahm
Rahm, masculine, ‘cream, crust of mould or mildew,’ from the equivalent Middle High German roum, masculine; compare Dutch room, Anglo-Saxon reám (earlier Modern English ream), Old Icelandic rjóme, ‘cream.’ The â of the Modern High German form compared with Old High German *roum (Thuringian roum) is dialectal (compare Middle High German strâm and stroum under Strom). The origin of these cognates has not yet been discovered.