The Collaborative International Dictionary of English v.0.48:

Rash \Rash\, n. [OF. rasche an eruption, scurf, F. rache; fr.
   (assumed) LL. rasicare to scratch, fr. L. radere, rasum, to
   scrape, scratch, shave. See Rase, and cf. Rascal.] (Med.)
   A fine eruption or efflorescence on the body, with little or
   no elevation.
   [1913 Webster]

   Canker rash. See in the Vocabulary.

   Nettle rash. See Urticaria.

   Rose rash. See Roseola.

   Tooth rash. See Red-gum.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Canker rash \Can"ker rash`\ (Med.)
   A form of scarlet fever characterized by ulcerated or putrid
   sore throat.
   [1913 Webster]