Merriam-Webster's Collegiate® Dictionary, 11th Edition
SCUM
I. noun
Etymology: Middle English, from Middle Dutch schum; akin to Old High German scūm foam
Date: 14th century
1. extraneous matter or impurities risen to or formed on the surface of a liquid often as a foul filmy covering, the scoria of metals in a molten state ; dross , a slimy film on a solid or gelatinous object,
2. refuse , a low, vile, or worthless person or group of people, scummy adjective
II. intransitive verb (scummed; scumming)
Date: 1661
to become covered with or as if with scum