Английский Этимологический словарь - rash (n.)

- "red spots on skin," 1709, from Fr. rache "a sore," from O.Fr. rache "ringworm," from V.L. *rasicare "to scrape," from L. rasus "scraped," pp. of radere "to scrape." The connecting notion is of itching. Sense of "any sudden outbreak or proliferation" first recorded 1820.