The Collaborative International Dictionary of English v.0.48:

Comfrey \Com"frey\, n. [Prob. from F. conferve, L. conferva, fr.
   confervere to boil together, in medical language, to heal,
   grow together. So called on account of its healing power, for
   which reason it was also called consolida.] (Bot.)
   A rough, hairy, perennial plant of several species, of the
   genus Symphytum.
   [1913 Webster]

   Note: A decoction of the mucilaginous root of the "common
         comfrey" (Symphytum officinale) is used in cough
         mixtures, etc.; and the gigantic "prickly comfrey"
         (Symphytum asperrimum) is somewhat cultivated as a
         forage plant.
         [1913 Webster]