SCUM, noun

1. The extraneous matter or impurities which rise to the surface of liquors in boiling or fermentation, or which form on the surface by other means. The word is also applied to the scoria of metals.

2. The refuse; the recrement; that which is vile or worthless.

The great and the innocent are insulted by the scum and refuse of the people.

Addison.

SCUM, verb transitive To take the scum from; to clear off the impure matter from the surface; to skim.

You that scum the molten lead. Dryden.