noun

• (uncountable) A layer of impurities that accumulates at the surface of a liquid (especially molten metal or water).
  "During smelting, scum rises to the surface and is then removed by the smelter."
• (uncountable) A greenish water vegetation (such as algae), usually found floating on the surface of ponds
  "These organisms form scum in large quantities."
• (uncountable, slang, chiefly US) Semen.
  "Horse, who was always talking about facts, said, “Man, that can't be scum, ’cause scum is white.”"
• (derogatory, slang) A reprehensible person or persons.
  "People who sell used-up pens are scum, just total low-lives."

verb

• (obsolete) To scour (the land, sea, etc.).
  "SOo by Merlyns aduys ther were sente fore rydars to skumme the Countreye / & they mette with the fore rydars of the north / and made hem to telle whiche wey the hooste cam / and thenne they told it to Arthur / and by kyng Ban and Bors counceill they lete brenne and destroye alle the contrey afore them there they shold ryde"
• (obsolete) To gather together, as scum.
  "A great majority of the members are scummed together from the Jacobinical dregs of former periods of the revolution."
• (video games, informal) To startscum or savescum.