(1):
(v.) 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.
(2):
(v.) refuse; recrement; anything vile or worthless.
(3):
(v. t.) To take the scum from; to clear off the impure matter from the surface of; to skim.
(4):
(v. t.) To sweep or range over the surface of.
(5):
(v. i.) To form a scum; to become covered with scum. Also used figuratively.