rash (adj.)
late 14c., "nimble, quick, vigorous" (early 14c. as a surname), a Scottish and northern word, perhaps from Old English -ræsc (as in ligræsc "flash of lightning") or one of its Germanic cognates, from Proto-Germanic *raskuz (cognates: Middle Low German rasch, Middle Dutch rasc "quick, swift," German rasch "quick, fast"). Related to Old English horsc "quick-witted." Sense of "reckless, impetuous, heedless of consequences" is attested from c.1500. Related: Rashly; rashness.
rash (n.)
"eruption of small red spots on skin," 1709, perhaps from French rache "a sore" (Old French rasche "rash, scurf"), from Vulgar Latin *rasicare "to scrape" (also source of Old Provençal rascar, Spanish rascar "to scrape, scratch," Italian raschina "itch"), from Latin rasus "scraped," past participle of radere "to scrape" (see raze). The connecting notion would be of itching. Figurative sense of "any sudden outbreak or proliferation" first recorded 1820.