Synonyms for Monotonous (same or very similar meaning)
WordNet sense 1 (sounded or spoken in a tone unvarying in pitch):
flat, monotone, monotonic
WordNet sense 2 (characterized by lack of variation in pitch, tone, or volume):
unmodulated
WordNet sense 3 (tediously repetitious or lacking in variety):
humdrum
WordNet sense 4 (lacking in liveliness or animation):
dull
From the ODE community, based on WordNet
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