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.
A greenish water vegetation (such as algae), usually found floating on the surface of ponds
These organisms form scum in large quantities.
The topmost liquid layer of a cesspool or septic tank.
(chiefly US) semen
A reprehensible person or persons.
People who sell used-up pens are scum, just total low-lives.