humdrum vs monotony

humdrum and monotony both are nouns.

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

As nouns, humdrum and monotony are synonyms defined as:

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

Other synonyms of humdrum include sameness.

humdrum (noun)	monotony (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

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