Source : Webster's Revised Unabridged Dictionary (1913)

Monotonous \Mo*not"o*nous\, a. [Gr. ?; mo`nos alone, single + ?
   tone. See {Tone}.]
   Uttered in one unvarying tone; continued with dull
   uniformity; characterized by monotony; without change or
   variety; wearisome. -- {Mo*not"o*nous*ly}, adv. --
   {Mo*not"o*nous*ness}, n.

Source : WordNet®

monotonously
     adv : in a monotonous manner; "the history of the play throughout
           the latter part of the eighteenth century is
           monotonously uneventful"