What is the meaning of Scum?

A layer of impurities that accumulates at the surface of a liquid (especially molten metal or water).

A greenish water vegetation (such as algae), usually found floating on the surface of ponds

The topmost liquid layer of a cesspool or septic tank.

Semen.

A reprehensible person or persons.

To remove the layer of scum from (a liquid etc.).

To remove (something) as scum.

To become covered with scum.

To scour (the land, sea, etc.).

To gather together, as scum.

To startscum or savescum.

Source: wiktionary.org