1326 (implied in scummer "shallow ladle for removing scum"), from M.Du. schume "foam, froth," from P.Gmc. *skuma- (cf. O.N. skum, O.H.G. scum, Ger. Schaum "foam, froth"), perhaps from PIE base *(s)keu- "to cover, conceal" (see
hide (n.1)). Sense deteriorated from "thin layer
atop liquid" to "film of dirt," then just "dirt." Meaning "lowest class of humanity" is 1586; scum of the Earth is from 1712. Adopted in Romanic, cf. O.Fr. escume, Mod.Fr. écume, Sp. escuma, It. schiuma. Adj. scummy first attested 1577; transf. sense of "filthy, disreputable" is recorded from 1932. Slang scumbag "condom" is from 1967; meaning "despicable person" is from 1971.