The Collaborative International Dictionary of English v.0.48:

Violaceous \Vi`o*la"ceous\, a. [L. violaceus, fr. viola a
   violet.]
   1. Resembling violets in color; bluish purple.
      [1913 Webster]

   2. (Bot.) Of or pertaining to a natural order of plants, of
      which the violet is the type. It contains about twenty
      genera and two hundred and fifty species.
      [1913 Webster]