An Etymological Dictionary of the German Language/Annotated/Ruß
Ruß, masculine, ‘soot,’ from the equivalent Middle High German and Old High German ruoȥ, masculine; compare Dutch roet, ‘soot.’ Low German sot, English soot, and its equivalent Anglo-Saxon sôt are scarcely allied. Old High German ruoȥ probably represents*hruoȥ,*hrôta-, but whether it is to be connected with Gothic hrôt, ‘roof,’ is more than questionable; it is more probably allied to Anglo-Saxon hrôt, ‘dirt.’ No cognate terms are found in the non-Teutonic languages.