early 14c., "froth, foam, thin layer atop liquid" (implied in scomour "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"), which is perhaps from PIE root *(s)keu- "to cover, conceal" on the notion of "that which covers the water."
Especially (late 14c.) "impure foam or extraneous substance that rises to the surface when liquid boils." Hence any sort of impure froth, and the sense deteriorated to "film of dirt," then simply "dirt, filth." The meaning "lowest class of humanity" is from 1580s; scum of the Earth is attested by 1712. The Germanic word was adopted in Romanic (Old French escume, Modern French écume, Spanish escuma, Italian schiuma). As a verb, "remove the scum from," late 14c.
Middle English fom, fome (c. 1300), from Old English fam "foam, saliva froth; sea," from West Germanic *faimo- (source also of Old High German veim, German Feim), from PIE root *(s)poi-mo- "foam, froth" (source also of Sanskrit phenah; Latin pumex "pumice," spuma "foam;" Old Church Slavonic pena "foam;" Lithuanian spainė "a streak of foam"). The plastic variety used in packaging, etc., so called from 1937.
mid-15c., "to foam, to froth," from cream (n.). From 1610s in figurative sense of "remove the best part of." Meaning "to beat, thrash, wreck" is 1929, U.S. slang; the exact sense connection is unclear. There was a slang cream (v.) in the 1920s that meant "cheat, deceive, especially by guile." Related: Creamed; creaming.
"cream," later also "cream-like froth on any liquid," a word now dialectal or obsolete, Old English ream, from Proto-Germanic *raumoz (source also of Middle Dutch and Dutch room, German Rahm), a word of uncertain origin. Related: Reamy.