Scum is a layer of dirt or foam on the top of a liquid.
Scum is usually thought of as very dirty and not good.
Examples:
There is scum on the pond.
The scum is on the top of the water.
The scum is very dirty.
The scum looks like a cloud on the water.
The scum is so thick, you can walk on it!
The scum is like a hat for the water.
Scum can also mean a person or group of people who are not liked or respected.
Calling someone scum is a very mean thing to do.
Examples:
He is a scum.
The scum is not welcome here.
The scum is not nice.
The scum is not good.
The scum is very bad.
The scum is like a bad smell.