- rash (adj.)
- c.1300, "nimble, quick, vigorous," a Scottish and northern word, perhaps from Old English -ræsc (cf. ligræsc "flash of lightning"), from Proto-Germanic *raskuz (cf. 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.
- rash (n.)
- "red spots on skin," 1709, perhaps from French rache "a sore," from Old French rache "ringworm," from Vulgar Latin *rasicare "to scrape" (cf. 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 is of itching. Sense of "any sudden outbreak or proliferation" first recorded 1820.