scumyoudaoicibaDictYouDict[scum 词源字典]
scum: [13] Scum is etymologically a ‘layer on top’ of something. The word’s modern connotations of ‘dirt’ are a secondary development. It comes ultimately from prehistoric Germanic *skūman, a derivative of the base *skū- ‘cover’, and its relatives include German schaum ‘foam’ (source of English meerschaum [18], literally ‘sea-foam’).

English scum originally meant ‘foam’ too (‘Those small white Fish to Venus consecrated, though without Venus’ aid they be created of th’ Ocean scum’, Joshua Sylvester, Divine Weeks and Works of Du Bartas 1598), the notion being of a layer of froth ‘covering’ liquid, but by the 15th century it was broadening out to any ‘film on top of liquid’, and from there it went downhill to a ‘film of dirt’ and then simply ‘dirt’.

Germanic *skūman was borrowed into Old French as escume, and this formed the basis of a verb escumer ‘remove the top layer’, from which English gets skim [15].
=> meerschaum, skim[scum etymology, scum origin, 英语词源]
scum (n.)youdaoicibaDictYouDict
early 14c. (implied in scummer "shallow ladle for removing scum"), from Middle Dutch schume "foam, froth," from Proto-Germanic *skuma- (cognates: Old Norse skum, Old High German scum, German Schaum "foam, froth"), perhaps from PIE root *(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 1580s; scum of the Earth is from 1712. Adopted in Romanic (Old French escume, Modern French écume, Spanish escuma, Italian schiuma).