Monotonous means:
* Lacking in variety and interest; dull, tedious, and unchanging.
Here are some synonyms for monotonous:
* Boring
* Repetitive
* Uninspiring
* Drab
* Bland
* Tiresome
* Humdrum
Example:
* "The speaker's monotonous voice lulled the audience to sleep."
* "She found her job to be incredibly monotonous, doing the same tasks every day."