Noun

1. A layer of impurities that accumulates at the surface of a liquid (especially molten metal or water).
2. A greenish water vegetation (such as algae), usually found floating on the surface of ponds
3. The topmost liquid layer of a cesspool or septic tank.
4. A person or persons considered to be reprehensible.

Synonyms

Verb

1. To remove the layer of scum from (a liquid etc.).
2. To scour (the land, sea etc.).
   • 1485, Sir Thomas Malory, Le Morte Darthur, Book I.13:

     Soo by Merlyns aduys ther were sente fore rydars to skumme the Countreye, & they mette with the fore rydars of the north [...].