scum bag

scum bag (http://definr.com/scum bag)

     n : a person who is deemed to be despicable or contemptible;
         "only a rotter would do that"; "kill the rat"; "throw the
         bum out"; "you cowardly little pukes!" [syn: rotter, rat,
          skunk, stinker, stinkpot, bum, puke, crumb,
         lowlife, so-and-so]