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]
varletyoudaoicibaDictYouDict
varlet: [15] Varlet and valet [16] are doublets – they come from the same ultimate source. This was Vulgar Latin *vassus, a borrowing from Old Celtic *wasso- ‘young man, squire’. From *vassus were derived two medieval Latin diminutive forms: vassallus, which has given English vassal [14], and *vassellitus. This passed into Old French as vaslet, which diversified into valet (source of English valet) and varlet (source of English varlet).

Both to begin with retained their original connotations of a ‘young man in service to a knight’, and hence by extension any ‘feudal retainer or servant’, but while valet still denotes a ‘servant’, varlet went down in the world in the 16th century to ‘knave’.
=> valet, vassal