scab n.

skin disease characterized by eruptions and inflammation, c. 1400, "the itch; scabby skin generally," from Latin scabies "mange, itch, roughness," from scabere "to scratch, scrape" (from PIE root *(s)kep-, a base forming words meaning "to cut, scrape, hack," source also of Gothic scaban, Old English sceafan "to scrape, shave;" Greek skaptein "to dig;" Old Church Slavonic skobli "scraper;" Lithuanian skabus "sharp," skabėti "to cut;" Lettish skabrs "splintery, sharp").

Modern medical use in reference to a contagious skin disease due to a parasitic mite is by 1814. The older name for a skin condition or disease was simply scab. Scabbed "afflicted with scabies or mange" is by c. 1300. Related: Scabious.

early 15c., scabbi, from scabbe (see scab (n.)).

1660s, of persons, "poorly dressed;" 1680s of clothes, furniture, etc., "of mean appearance, no longer new or fresh;" with -y (2) + shab "a low fellow" (1630s), extended from the original sense, "scabies." This is from Middle English shabbe "skin disease characterized by eruptions, itching, etc.," from Old English sceabb, the native form of Modern English scab (n.), which was influenced by Scandinavian (see sh-).

Shab (n.) survives in reference to a disease of sheep, but in Middle English shabbed meant "suffering from scabies, mange, etc." (from Old English sceabbed).

Shabby in the sense of "inferior in quality" is from 1805. The figurative meaning "contemptibly mean" is from 1670s. Similar formation in Middle Dutch schabbich, German schäbig "shabby." Related: Shabbily; shabbiness. Carlyle has shabbish "somewhat shabby."

Shabby-genteel "run-down but trying to keep up appearances, retaining in present shabbiness traces of former gentility," is attested by 1754. Shabaroon, shabberoon "disreputable person" is attested from c. 1700.