Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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}, {scurvy}]
     2: covered with scum; "the scummy surface of the polluted pond"