The Collaborative International Dictionary of English v.0.48:

Ethereous \E*the"re*ous\, a. [L.aethereus, Gr. ? See Ether.]
   1. Formed of ether; ethereal. [Obs.]
      [1913 Webster]

            This ethereous mold whereon we stand. --Milton.
      [1913 Webster]

   2. (Chem.) Pertaining to, or resembling, ether.
      [1913 Webster]

   Ethereous oil. See Ethereal oil, under Ethereal.
      [1913 Webster]