Scum (noun)

1. A layer of impurities that forms on the surface of a liquid, especially when it is heated.
2. A person or group of people considered worthless or contemptible.

Origin:

Middle english (denoting a thin layer of impurities on a liquid): from old norse skum "foam, scum".

Examples:

1. The scum on the surface of the soup needed to be removed.
2. The scum on the pond was a breeding ground for mosquitoes.
3. He was a scum who took advantage of the vulnerable.
4. The society was full of scum who preyed on the weak.
5. The city was full of scum who were just out for themselves.