skin and bones vs someone

skin and bones and someone both are nouns.

Word	Noun	Adjective	Verb	Adverb
skin and bones	Yes	No	No	No
someone	Yes	No	No	No

As nouns, someone is a hypernym of skin and bones; that is, someone is a word with a broader meaning than skin and bones:

skin and bones: 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 skin and bones include individual, mortal, person, somebody, soul.

skin and bones (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