Scum is a type of dirty or foul matter that's on the surface of something like water or mud. When you look at a stagnant pond, you might see scum floating on top.
You'll often hear people use the word "scum" to describe people who behave badly or are considered low-class. Think of it as referring to someone who leaves trash behind wherever they go – not just physical trash but also someone with bad manners or no respect for others.
How common is "scum"?
Word scum is considered rare in modern English. It appears most frequently in fiction texts and less often in other writings.
Definitions
noun
A layer of impurities that accumulates at the surface of a liquid (especially molten metal or water).
Example: During smelting, scum rises to the surface and is then removed by the smelter.
A greenish water vegetation (such as algae), usually found floating on the surface of ponds
Example: These organisms form scum in large quantities.
The topmost liquid layer of a cesspool or septic tank.
(chiefly US) semen
A reprehensible person or persons.
Example: People who sell used-up pens are scum, just total low-lives.