subaltern vs lowly

subaltern is a noun but lowly is not a noun.

Word	Noun	Adjective	Verb	Adverb
subaltern	Yes	Yes	No	No
lowly	No	Yes	No	No

As adjectives, subaltern and lowly are synonyms defined as:

Other synonyms of subaltern include junior-grade, lower-ranking, petty, secondary.

subaltern (noun)	lowly (noun)
a British commissioned army officer below the rank of captain

subaltern (adjective)	lowly (adjective)
inferior in rank or status	low or inferior in station or quality
	inferior in rank or status
	of low birth or station (`base' is archaic in this sense)
	used of unskilled work (especially domestic work)