The Collaborative International Dictionary of English v.0.48:

Ethionic \Eth`i*on"ic\, a. [Ethyl + thionic.] (Chem.)
   Pertaining to, derived from, or designating, an acid so
   called.
   [1913 Webster]

   Ethionic acid (Chem.), a liquid derivative of
      ethylsulphuric and sulphuric (thionic) acids, obtained by
      the action of sulphur trioxide on absolute alcohol. Ethiop