1. The extraneous matter or impurities which rise to the surface of liquids in boiling or fermentation, or which form on the surface by other means; also, the scoria of metals in a molten state; dross. Some to remove the scum it did rise. (Spenser)
2. Refuse; recrement; anything vile or worthless. The great and innocent are insulted by the scum and refuse of the people. (Addison)
Origin: Of Scand. Origin; cf. Dan. & Sw. Skum, Icel. Skm, LG. Schum, D. Schuim, OHG. Scm, G. Schaum; probably from a root meaning, to cover. Cf. Hide skin, Meerschaum, Skim, Sky.