poor white trash vs Caucasian

poor white trash and Caucasian both are nouns.

Word	Noun	Adjective	Verb	Adverb
poor white trash	Yes	No	No	No
Caucasian	Yes	Yes	No	No

As nouns, Caucasian is a hypernym of poor white trash; that is, Caucasian is a word with a broader meaning than poor white trash:

poor white trash: (slang) an offensive term for White people who are impoverished
Caucasian: a person of European descent with a light-skinned or pale complexion

Other hypernyms of poor white trash include White, White person, have-not, poor person.

poor white trash (noun)	Caucasian (noun)
(slang) an offensive term for White people who are impoverished	a number of languages spoken in the Caucasus that are unrelated to languages spoken elsewhere
	a person of European descent with a light-skinned or pale complexion

poor white trash (adjective)	Caucasian (adjective)
	of or relating to the geographical region of Caucasia
	of or relating to Caucasian people