thin person vs someone

thin person and someone both are nouns.

Word	Noun	Adjective	Verb	Adverb
thin person	Yes	No	No	No
someone	Yes	No	No	No

As nouns, someone is a hypernym of thin person; that is, someone is a word with a broader meaning than thin person:

thin person: a person who is unusually thin and scrawny
someone: a human being; person, singular, assertive existential pronoun; pronoun, person, singular; quantifier: assertive existential

Other hypernyms of thin person include individual, mortal, person, somebody, soul.

thin person (noun)	someone (noun)
a person who is unusually thin and scrawny	a human being; person, singular, assertive existential pronoun; pronoun, person, singular; quantifier: assertive existential