scum (n.)

early 14c. (implied in scummer "shallow ladle for removing scum"), from Middle Dutch schume "foam, froth," from Proto-Germanic *skuma- (source also of Old Norse skum, Old High German scum, German Schaum "foam, froth"), perhaps from PIE root *(s)keu- "to cover, conceal" on the notion of "that which covers the water."

Sense deteriorated from "thin layer atop liquid" to "film of dirt," then just "dirt." Meaning "lowest class of humanity" is 1580s; scum of the Earth is from 1712. Adopted in Romanic (Old French escume, Modern French écume, Spanish escuma, Italian schiuma).

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Definitions of scum from WordNet
scum (n.)
worthless people;
Synonyms: trash
scum (n.)
a film of impurities or vegetation that can form on the surface of a liquid;
scum (v.)
remove the scum from;