rash adj.

1540s, "completely destroy," an alteration of racen "pull or knock down" (a building or town), from earlier rasen (14c.), etymologically "to scratch, slash, scrape, erase," from Old French raser "to scrape, shave," from Medieval Latin rasare, frequentative of Latin radere (past participle rasus) "to scrape, shave." This has cognates in Welsh rhathu, Breton rahein "to scrape, shave." Watkins says it is "possibly" from an extended form of the PIE root *red- "to scrape, scratch, gnaw." But de Vaan writes, "Since this word family is only found in Italo-Celtic, a PIE origin is uncertain." From 1560s as "shave off, remove by scraping," also "cut or wound slightly, graze." Related: Razed; razing.

mid-14c., rascaile "people of the lowest class, the general mass; rabble or foot-soldiers of an army" (senses now obsolete), also singular, "low, tricky, dishonest person," from Old French rascaille "rabble, mob" (12c., Modern French racaille), as Cotgrave's French-English Dictionary (1611) defines it: "the rascality or base and rascall sort, the scumme, dregs, offals, outcasts, of any company."

This is of uncertain origin, perhaps a diminutive from Old French rascler, from Vulgar Latin *rasicare "to scrape" (see rash (n.)) on the notion of "the scrapings." "[U]sed in objurgation with much latitude, and often, like rogue, with slight meaning" [Century Dictionary]. Used also in Middle English of animals unfit to chase as game on account of some quality, especially a lean deer. Also formerly an adjective.

*rēd-, Proto-Indo-European root meaning "to scrape, scratch, gnaw."

It forms (possibly) all or part of: abrade; abrasion; corrode; corrosion; erase; erode; erosion; radula; rascal; rase; rash (n.) "eruption of small red spots on skin;" raster; rat; raze; razor; rodent; rostrum; tabula rasa.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit radati "scrapes, gnaws," radanah "tooth;" Latin rodere "to gnaw, eat away," radere "to scrape;" Welsh rhathu "scrape, polish."