The Collaborative International Dictionary of English v.0.48:

Arenation \Ar`e*na"tion\ ([a^]r`[-e]*n[=a]"sh[u^]n), n. [L.
   arenatio, fr. arena sand.] (Med.)
   A sand bath; application of hot sand to the body.
   --Dunglison.
   [1913 Webster]