scum

Concise Oxford English Dictionary © 2008 Oxford University Press:
scum/skʌm/

▶noun

• 1 a layer of dirt or froth on the surface of a liquid.

• 2 informal a worthless or contemptible person or group of people.

▶verb (scums, scumming, scummed) cover or become covered with a layer of scum.

– derivatives
scummy adjective.

– origin ME: from Mid. Low Ger., MDu. schūm, of Gmc origin.