Etymology
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scum (n.)

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.

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scummer (n.)

"implement used in skimming," early 14c., scomour; see scum (n.) + -er.

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scummy (adj.)

"covered with scum," 1570s, from scum + -y (2). Transferred sense of "filthy, disreputable" is recorded from 1932. Related: Scumminess.

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scumbag (n.)

by 1939, "condom," slang, from scum + bag (n.). Earlier (by 1817) it was used in sugar refining as the name of a frame covered in coarse cloth used in straining. The meaning "despicable person" is attested by 1971.

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skim (v.)
early 15c. (skimmer, the utensil, is attested from late 14c.), "to clear (a liquid) from matter floating on the surface, lift the scum from," from Old French escumer "remove scum," from escume (Modern French écume) "scum," from a Germanic source (compare Old High German scum "scum," German Schaum; see scum). Meaning "to throw (a stone) so as to skip across the surface of (water) is from 1610s. Meaning "to move lightly and rapidly over the surface of" is from 1650s, from the motion involved in skimming liquid; that of "to glance over carelessly" (in reference to printed matter) recorded by 1799. Related: Skimmed; skimming.
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*(s)keu- 

Proto-Indo-European root meaning "to cover, conceal."

It forms all or part of: chiaroscuro; cunnilingus; custody; cutaneous; cuticle; -cyte; cyto-; hide (v.1) "to conceal;" hide (n.1) "skin of a large animal;" hoard; hose; huddle; hut; kishke; lederhosen; meerschaum; obscure; scum; skewbald; skim; sky.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit kostha "enclosing wall," skunati "covers;" Greek kytos "a hollow, vessel," keutho "to cover, to hide," skynia "eyebrows;" Latin cutis "skin," ob-scurus "dark;" Lithuanian kiautas "husk," kūtis "stall;" Armenian ciw "roof;" Russian kishka "gut," literally "sheath;" Old English hyd "a hide, a skin," hydan "to hide, conceal; Old Norse sky "cloud;" Old English sceo "cloud;" Middle High German hode "scrotum;" Old High German scura, German Scheuer "barn;" Welsh cuddio "to hide."

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mother (n.2)

"a thick substance concreting in liquors; the lees or scum concreted" [Johnson], 1530s, probably from Middle Dutch modder "filth, dregs," from PIE *meu- (see mud).

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meerschaum (n.)

type of soft white clay, 1784; from 1789 as "tobacco pipe with a bowl made of baked meerschaum clay," from German Meerschaum, literally "sea-foam," so called from its frothy appearance; from Old High German mari "sea" (see mere (n.1)) + scum "scum" (see skim (v.)). A loan-translation of Latin spuma maris, itself said to be a loan translation of Greek halos akhne, from Persian kaf-i-darya.

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recrement (n.)

"dross, scum, superfluous matter, separated from that which is useful," especially a waste product of an animal or vegetable body, 1590s, from French récrément (mid-16c.) or directly from Latin recrementum, as if from a verb *recernere, from re- (see re-) + cernere "to sift, separate" (from PIE root *krei- "to sieve," thus "discriminate, distinguish"). Related: Recremental (1570s); recrementitious.

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ooze (v.)

"to flow as ooze, percolate through the pores of a substance" (intrans.), also "emit in the shape of moisture" (trans.), late 14c., wosen, verbal derivative of Old English noun wos "juice, sap," from Proto-Germanic *wosan (source of Middle Low German wose "scum"), from same source as ooze (n.). The modern spelling is from late 16c. The Old English verb was wesan. Related: Oozed; oozing.

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