scum

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A filmy layer of extraneous or impure matter that forms on or rises to the surface of a liquid or body of water.
2. n. The refuse or dross of molten metals.
3. n. Refuse or worthless matter.
4. n. Slang One, such as a person or an element of society, that is regarded as despicable or worthless.
5. v. To remove the scum from.
6. v. To become covered with scum.

Century Dictionary and Cyclopedia

1. n. Foam; froth: as, the scum of the sea.
2. n. The impurities or extraneous substances which rise to the surface of liquids, as in boiling or fermentation, or which form by other means; also, the scoria of molten metals; hence, by extension, any film or surface of foul floating matter: as, the scum of a stagnant pond.
3. n. Refuse; dross; offscourings.
4. To remove the scum from; clear off the froth, dross, or impurities that have risen to or formed on the surface of; skim.
5. To sweep over; move swiftly upon; skim.
6. To arise or be formed on the surface as foam or scum; be thrown up as scum.
7. To be or become covered with scum: generally with over.
8. To skim lightly: with over.

Wiktionary

1. n. uncountable A layer of impurities that accumulates at the surface of a liquid (especially molten metal or water).
2. n. uncountable A greenish water vegetation (such as algae), usually found floating on the surface of ponds
3. n. The topmost liquid layer of a cesspool or septic tank.
4. n. countable (derogatory, slang) A reprehensible person or persons.
5. v. To remove the layer of scum from (a liquid etc.).
6. v. To remove (something) as scum.
7. v. obsolete To scour (the land, sea etc.).

GNU Webster's 1913

1. n. The extraneous matter or impurities which rise to the surface of liquids in boiling or fermentation, or which form on the surface by other means; also, the scoria of metals in a molten state; dross.
2. n. refuse; recrement; anything vile or worthless.
3. v. To take the scum from; to clear off the impure matter from the surface of; to skim.
4. v. obsolete To sweep or range over the surface of.
5. v. To form a scum; to become covered with scum. Also used figuratively.

WordNet 3.0

1. n. worthless people
2. v. remove the scum from
3. n. a film of impurities or vegetation that can form on the surface of a liquid