"red spots on skin," 1709, perhaps from Fr. rache "a sore," from O.Fr. rache "ringworm," from V.L. *rasicare "to scrape" (cf. O.Prov. rascar, Sp. rascar "to scrape, scratch," It. raschina "itch"), from L. rasus "scraped," pp. of radere "to scrape" (see raze). The connecting notion is of itching. Sense of "any sudden outbreak or proliferation" first recorded 1820.
[rash] 
