n.
  - a layer of dirt or froth on the surface of a liquid.
 - informal a worthless or contemptible person or group of people.
 
v.
 (scums, scumming, scummed)
 cover or become covered with a layer of scum.
 
  Derivative
  
  Etymology
  ME: from Mid. Low Ger., MDu. schūm, of Gmc origin.