skin and bones vs somebody

skin and bones and somebody both are nouns.

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

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

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

Other hypernyms of skin and bones include individual, mortal, person, someone, soul.

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