Scum is the layer of dirt, foam, or other unpleasant material that forms on top of a liquid. When you see a grimy film floating on the surface of a pond or notice soap residue around the edge of a bathtub, that's scum. It refers to the unwanted stuff that rises to the top.

In cooking, scum might appear on top of a pot of boiling soup or stock. Cooks often skim it off with a spoon to keep the liquid clear and clean. In nature, scum can form on stagnant water where algae and bacteria grow together into a greenish coating.

People also use scum as a harsh insult for someone they consider completely worthless or morally disgusting. When someone calls another person scum, they're saying that person has behaved so badly that they deserve contempt. This usage is quite strong and mean, so it's not something you'd use lightly. You might hear it in movies when a character expresses anger at someone who has done something truly rotten, like “That villain is the scum of the earth.”

The adjective form is scummy, which describes either something covered in actual scum or behavior that's sleazy and dishonest.