1. A filmy layer of extraneous or impure matter that forms on or rises to the surface of a liquid or body of water.
2. The refuse or dross of molten metals.
3. Refuse or worthless matter.
4. Slang. One, such as a person or an element of society, that is regarded as despicable or worthless.

v., scummed, scum·ming, scums.

v.tr.

To remove the scum from.

v.intr.

To become covered with scum.

[Middle English, from Middle Dutch schûm.]

scum'mer n.
scum'mi·ly adv.
scum'mi·ness n.
scum'my adj.

Source: The American Heritage® Dictionary of the English Language, Fourth Edition
Copyright © 2000 by Houghton Mifflin Company.
Published by Houghton Mifflin Company.