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®

monotonous
     adj 1: sounded or spoken in a tone unvarying in pitch; "the owl's
            faint monotonous hooting" [syn: {monotone}, {monotonic}]
     2: tediously repetitious or lacking in variety; "a humdrum
        existence; all work and no play"; "nothing is so
        monotonous as the sea" [syn: {humdrum}]