skin and bones vs thin person

skin and bones and thin person both are nouns.

Word	Noun	Adjective	Verb	Adverb
skin and bones	Yes	No	No	No
thin person	Yes	No	No	No

As nouns, skin and bones and thin person are synonyms defined as:

skin and bones and thin person: a person who is unusually thin and scrawny

Other synonyms of skin and bones include scrag.

skin and bones (noun)	thin person (noun)
a person who is unusually thin and scrawny	a person who is unusually thin and scrawny