Scum
Middle English: from Middle Low German, Middle Dutch schūm, of Germanic origin.
wiktionary
From Middle English scum, scome, skum, skome, scumme, from Middle Dutch schūme(“foam”), from Proto-Germanic *skūmaz(“froth, foam”), from Proto-Indo-European *skew-(“to cover, conceal”). Cognate with Dutch schuim(“foam”), German Schaum(“foam”), Danish and Swedish skum(“foam”). Compare also French écume(“scum”), Italian schiuma(“foam”), Walloon schome(“scum, foam”), Lithuanian šamas(“catfish”) and skanus(“tasty”) from the same Germanic source. Related to skim.
etymonline
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).
