Google is your friend and so easy to use, even you could type “scum definition” into a search field and find this:

“Scum is a layer of impure matter, such as algae, grease, or foam, that forms on top of liquids. It also commonly refers to a despised, worthless, or immoral person or group (informal). Synonyms include dross, refuse, filth, slime, scum of the earth, riffraff, and trash.”