Merriam-Webster Collegiate Dictionary - scum

 I. noun  Etymology: Middle English, from Middle Dutch schum; akin to Old High German scum foam  Date: 14th century  1.  a. extraneous matter or impurities risen to or formed on the surface of a liquid often as a foul filmy covering — compare pond ~ 2  b. the scoria of metals in a molten state ; dross  c. a slimy film on a solid or gelatinous object  2.  a. refuse  b. a low, vile, or worthless person or group of people  • ~my adjective  II. intransitive verb  (~med; ~ming)  Date: 1661 to become covered with or as if with ~