The Collaborative International Dictionary of English v.0.48:

Scum \Scum\ (sk[u^]m), n. [Of Scand. origin; cf. Dan. & Sw.
   skum, Icel. sk[=u]m, LG. schum, D. schuim, OHG. sc[=u]m, G.
   schaum; probably from a root meaning, to cover. [root]158.
   Cf. Hide skin, Meerschaum, Skim, v., Sky.]
   [1913 Webster]
   1. 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.
      [1913 Webster]

            Some to remove the scum as it did rise. --Spenser.
      [1913 Webster]

   2. refuse; recrement; anything vile or worthless.
      [1913 Webster]

            The great and innocent are insulted by the scum and
            refuse of the people.                 --Addison.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Scum \Scum\, v. t. [imp. & p. p. Scummed; p. pr. & vb. n.
   Scumming.]
   1. To take the scum from; to clear off the impure matter from
      the surface of; to skim.
      [1913 Webster]

            You that scum the molten lead.        --Dryden &
                                                  Lee.
      [1913 Webster]

   2. To sweep or range over the surface of. [Obs.]
      [1913 Webster]

            Wandering up and down without certain seat, they
            lived by scumming those seas and shores as pirates.
                                                  --Milton.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Scum \Scum\, v. i.
   To form a scum; to become covered with scum. Also used
   figuratively.
   [1913 Webster]

         Life, and the interest of life, have stagnated and
         scummed over.                            --A. K. H.
                                                  Boyd.
   [1913 Webster]

WordNet (r) 3.0 (2006):

scum
    n 1: worthless people [syn: trash, scum]
    2: a film of impurities or vegetation that can form on the
       surface of a liquid
    v 1: remove the scum from

Moby Thesaurus II by Grady Ward, 1.0:

239 Moby Thesaurus words for "scum":
   aerate, alluvion, alluvium, androcyte, antheridium, antherozoid,
   apply to, ash, beat, bilge, bilgewater, blanket, block, bones,
   breakers, canaille, canopy, carrion, cattle, chaff, cinder,
   clinker, cloak, clothe, cloud, coat, coating, collar, collop, cope,
   cover, cover up, covering, cowl, cream, culm, cur, curtain, cut,
   deadwood, deal, deposition, deposits, diluvium, dishwater, disk,
   ditchwater, draff, dregs, dregs of society, dross, dust, eclipse,
   ember, enamel, facing, feces, feuille, filings, film, flap, foam,
   foil, fold, froth, fur, garbage, gash, glop, grounds, gunk, head,
   hogwash, hood, husks, lacquer, lamella, lamina, laminated glass,
   laminated wood, lap, lather, lay on, lay over, leaf, leavings,
   lees, loess, male gamete, mantle, mask, masses, membrane, meringue,
   milt, mire, mob, moraine, mousse, muck, muffle, obduce, obscure,
   occult, offal, offscourings, offscum, ooze, orts, overlay,
   overspread, paint, pane, panel, parings, patina, peel, pellicle,
   plait, plank, plate, plating, ply, plywood, pollen, potsherds,
   precipitate, precipitation, prick, proletariat, protein, puff,
   put on, raff, rags, ragtag and bobtail, rasher, raspings, refuse,
   revetment, riffraff, rubbish, safety glass, scale, scoria,
   scourings, scrap iron, scraps, screen, scud, scurf, sea foam,
   sediment, seed, semen, seminal fluid, settlings, sewage, sewerage,
   shards, shavings, sheet, shield, shit, shithead, silt, sinter,
   skin, skunk, slab, slack, slag, slat, slice, slime, slob, slop,
   slops, slosh, slough, sludge, slush, smut, snake, soapsuds, soot,
   sordes, souffle, sperm, sperm cell, spermagonium, spermatic fluid,
   spermatid, spermatiophore, spermatium, spermatocyte,
   spermatogonium, spermatophore, spermatozoa, spermatozoid,
   spermatozoon, spindrift, splosh, spoondrift, spray, spread over,
   spume, stinging, stinker, stubble, sublimate, sud, suds,
   superimpose, superpose, surf, sweepings, swill, swinish multitude,
   table, tablet, tares, toad, trash, turd, unwashed, varnish, veil,
   veneer, vermin, wafer, wastage, waste, waste matter, wastepaper,
   weeds, whip, whisk, white water