noun

1. 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.”

2. A greenish water vegetation (such as algae), usually found floating on the surface of ponds

   “These organisms form scum in large quantities.”

3. The topmost liquid layer of a cesspool or septic tank.

Synonyms: dross, impurities, cinder, scoriae, slag, bastard

verb

1. To remove the layer of scum from (a liquid etc.).

2. To remove (something) as scum.

3. To become covered with scum.