scummer 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 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.

English agent noun ending, corresponding to Latin -or. In native words it represents Old English -ere (Old Northumbrian also -are) "man who has to do with," from Proto-Germanic *-ari (cognates: German -er, Swedish -are, Danish -ere), from Proto-Germanic *-arjoz. Some believe this root is identical with, and perhaps a borrowing of, Latin -arius (see -ary).

Generally used with native Germanic words. In words of Latin origin, verbs derived from past participle stems of Latin ones (including most verbs in -ate) usually take the Latin ending -or, as do Latin verbs that passed through French (such as governor); but there are many exceptions (eraser, laborer, promoter, deserter; sailor, bachelor), some of which were conformed from Latin to English in late Middle English.

The use of -or and -ee in legal language (such as lessor/lessee) to distinguish actors and recipients of action has given the -or ending a tinge of professionalism, and this makes it useful in doubling words that have a professional and a non-professional sense (such as advisor/adviser, conductor/conducter, incubator/incubater, elevator/elevater).