underclass vs lower class

underclass and lower class both are nouns.

Word	Noun	Adjective	Verb	Adverb
underclass	Yes	Yes	No	No
lower class	Yes	No	No	No

As nouns, underclass and lower class are synonyms defined as:

underclass and lower class: the social class lowest in the social hierarchy

underclass (noun)	lower class (noun)
the social class lowest in the social hierarchy	the social class lowest in the social hierarchy

underclass (adjective)	lower class (adjective)
belonging to the lowest and least privileged social stratum