Source : Webster's Revised Unabridged Dictionary (1913)

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.]