The Collaborative International Dictionary of English v.0.48:

Rhetian \Rhe"ti*an\, a. [L. Rhaetius, Raetius: cf. F.
   rh['e]tien.]
   Pertaining to the ancient Rhaeti, or Rhaetians, or to
   Rhaetia, their country; as, the Rhetian Alps, now the country
   of Tyrol and the Grisons.
   [1913 Webster]