The Collaborative International Dictionary of English v.0.48:

Scummy \Scum"my\, a.
   Covered with scum; of the nature of scum. --Sir P. Sidney.
   [1913 Webster]

WordNet (r) 3.0 (2006):

scummy
    adj 1: of the most contemptible kind; "abject cowardice"; "a low
           stunt to pull"; "a low-down sneak"; "his miserable
           treatment of his family"; "You miserable skunk!"; "a
           scummy rabble"; "a scurvy trick" [syn: abject, low,
           low-down, miserable, scummy, scurvy]
    2: covered with scum; "the scummy surface of the polluted pond"

Moby Thesaurus II by Grady Ward, 1.0:

125 Moby Thesaurus words for "scummy":
   abject, abominable, arrant, atrocious, barfy, base, beggarly,
   beneath contempt, cheap, cheesy, common, contemptible, crappy,
   crummy, debased, degraded, depraved, despicable, dirty, disgusting,
   execrable, fecal, feculent, fetid, filmy, filthy, flagrant,
   flyblown, foliaceous, foliated, foul, fulsome, gaudy, gimcracky,
   gloppy, grave, gross, gunky, heinous, icky, in layers, lamellar,
   lamellate, lamellated, lamelliform, laminate, laminated, laminous,
   leaflike, leafy, little, low, low-down, lumpen, maggoty,
   malodorous, mangy, mean, measly, membranous, mephitic,
   meretricious, miserable, monstrous, mucky, nasty, nauseating,
   nefarious, obnoxious, odious, ordurous, paltry, pathetic, petty,
   pitiable, pitiful, poky, poor, puky, putrid, rank, reptilian,
   repulsive, rotten, rubbishy, sad, scabby, scrubby, scruffy, scurfy,
   scurvy, scuzzy, shabby, shitty, shoddy, slabby, slimy, sloppy,
   sloshy, sludgy, slushy, small, sorry, spathic, spathose, sposhy,
   squalid, stratified, stratiform, three-ply, trashy, trumpery,
   two-for-a-cent, two-for-a-penny, two-ply, twopenny,
   twopenny-halfpenny, unmentionable, valueless, vile, vomity, wormy,
   worthless, wretched, yecchy