The Collaborative International Dictionary of English v.0.48:

Ultroneous \Ul*tro"ne*ous\, a. [L. ultroneus, from ultro to the
   further side, on his part, of one's own accord. See
   Ultra-.]
   Spontaneous; voluntary. [Obs.] --Jer. Taylor. --
   Ul*tro"ne*ous*ly, adv. [Obs.] -- Ul*tro"ne*ous*ness, n.
   [Obs.]
   [1913 Webster]