Meaning : A person who is deemed to be despicable or contemptible.
Example :
Only a rotter would do that.
Kill the rat.
Throw the bum out.
You cowardly little pukes!.
The British call a contemptible person a `git'.
Synonyms : bum, crumb, dirty dog, git, lowlife, puke, rat, scum bag, skunk, so-and-so, stinker, stinkpot