scabies (n.)

mid-13c., in a general sense, "skin disease, 'the itch,' " developed from Old English sceabb (related to scafan "to shave, scrape, scratch") and from its cognate, Old Norse skabb, both from Proto-Germanic *skab- "scratch, shave" (from PIE *(s)kep- "to cut, scrape, hack;" see scabies). Likely reinforced by resemblance of the plural to Latin cognate scabies "scab, itch, mange" (from scabere "to scratch").

The word was extended late 14c. to the patches or ulcerations accompanying the disease, hence the main modern meaning "crust which forms over a wound or sore," attested by c. 1400.

The colloquial meaning "strikebreaker" is recorded by 1806, from earlier colloquial sense of "person who refuses to join a trade union" (1777), probably from meaning "despicable person; mean, paltry fellow" (1580s), which, according to OED, is possibly from Dutch, where a similar sense had developed. The flood-scoured scablands of the Pacific Northwest were so called by 1923.

1570s, "harsh, unmusical" (implied in scabrously), from Late Latin scabrosus "rough," from Latin scaber "rough, scaly," related to scabere "to scratch, scrape" (from PIE *(s)kep- "to cut, scrape, hack;" see scabies).

The sense in English evolved to "vulgar" (by 1881), "squalid" (by 1939), and "nasty, repulsive" (by 1951). The etmological sense of "rough, rugged, having little sharp points" is attested in English from 1650s. Related: Scabrousness.