The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]