thin person vs skin and bones

thin person and skin and bones both are nouns.

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

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

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

Other synonyms of thin person include scrag.

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