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]

WordNet (r) 3.0 (2006):

monotonously
    adv 1: in a monotonous manner; "the history of the play
           throughout the latter part of the eighteenth century is
           monotonously uneventful"

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "monotonously":
   ad infinitum, again and again, at a stretch, boresomely, boringly,
   ceaselessly, connectedly, constantly, continually, continuously,
   cumulatively, cyclically, drearisomely, dully, endlessly, equably,
   evenly, everlastingly, fatiguingly, harpingly, in a rut,
   incessantly, interminably, irksomely, on a stretch, on and on,
   on end, perennially, repetitively, round the clock, routinely,
   tediously, tiresomely, together, unbrokenly, unceasingly,
   uniformly, unintermittently, uninterruptedly, unrelievedly,
   unvaryingly, wearisomely, wearyingly, without a break,
   without cease, without stopping