monotony vs humdrum

monotony and humdrum both are nouns.

Word	Noun	Adjective	Verb	Adverb
monotony	Yes	No	No	No
humdrum	Yes	Yes	No	No

As nouns, monotony and humdrum are synonyms defined as:

monotony and humdrum: the quality of wearisome constancy, routine, and lack of variety

Other synonyms of monotony include sameness.

monotony (noun)	humdrum (noun)
the quality of wearisome constancy, routine, and lack of variety	the quality of wearisome constancy, routine, and lack of variety
constancy of tone or pitch or inflection

monotony (adjective)	humdrum (adjective)
	tediously repetitious or lacking in variety
	not challenging; dull and lacking excitement