The Collaborative International Dictionary of English v.0.48:

Rheic \Rhe"ic\, a. [NL. Rheum rhubarb, Gr. ??? See Rhubarb.]
   (Chem.)
   Pertaining to, or designating, an acid (commonly called
   chrysophanic acid) found in rhubarb (Rheum). [Obsoles.]
   [1913 Webster]