The word "monotonous" is an adjective.
with
The stem of the word monotonous is monotony.
The singer had an monotonous voice, the song was boring
Her voice was very monotonous, and I soon became bored listening to her talk.
Sally work and a cook show and became monotonous and she had fun cooking
Anything longer than the first round just becomes monotonous.
Answering all these simple questions is getting a bit monotonous
The noun form of the adjective 'monotonous' is monotonousness.A related noun form is monotony.
One possible adverb for "monotonous" is "monotonously," which describes an action done in a monotonous manner.
monotonously
monotonously