Uttered in one unvarying tone; continued with dull uniformity; characterized by monotony; without change or variety; wearisome.

sounded or spoken in a tone unvarying in pitch; "the owl's faint monotonous hooting"

tediously repetitious or lacking in variety; "a humdrum existence; all work and no play"; "nothing is so monotonous as the sea"

when all moustaches start to look the same