It is hard to define the word, but it is basically used to describe someone so disgraceful that they are seen as the lowest form of life. "Worthlessness", "waste of skin", "dirt". "Nothing". Far worse than most other insults, where the victim is often just referred to as genetalia.
The fact that "scum" is a non-swear, and seen as perfectly acceptable English, makes it even more demeaning and offensive. If you are condemned as a "cunt" or a "fucktard", then you are simply being attacked with cliché profanity, used spontaneously and with no particular venom or thought. "Scum", however, is usually only used in exclusive conditions where it is the only word for the job, and makes the victim feel like the lowest of the low.
2. n. Supporters or playing staff of an opposition football team, generally of a local rival
Fuck off Rovers Scum, I fuckin HATE ye
II. (n./ adj.) Sub-species of human being; often recognized due to severe lack of intelligence, painful fashion-sense and extreme levels of sloth.
1/. Those lower life-forms that sport burberry and/or ridiculous amounts of Nike brand clothing. This family of scum is often referred to as chav.
2/. Anybody who uses the word 'nigga' or makes a grunt sounding similar, but not exclusive to: 'blud' or 'shank' is automatically referred to by any decent human being as scum.
3/. Anything that kisses its teeth more than once a day.
4/. The vast majority of creatures inhabiting George Green's.
5/. Anything with an I.Q. lower than 50 (or supports Tottenham F.C.)
2. Scum: "Izzit coz I'm a nigga? I'll shank you, blud!"
Civilized human: "No, it is because you are scum! Good day, sir"
3. "(kisses teeth) Bombaclat!" = Scum
4. "Did you see that scum-sucking, stinky, fuck-bag?"
"Yeah, 'round here we call them yobs!"