The Collaborative International Dictionary of English v.0.48:

Caesious \C[ae]"si*ous\ (s[=e]"z[-e]*[u^]s), a. [L. caesius
   bluish gray.] (Nat. Hist.)
   Of the color of lavender; pale blue with a slight mixture of
   gray. --Lindley.
   [1913 Webster] caesium