spindleshanks vs skin and bones

spindleshanks and skin and bones both are nouns.

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

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

Other hypernyms of spindleshanks include scrag, thin person.

spindleshanks (noun)	skin and bones (noun)
long thin legs	a person who is unusually thin and scrawny
a thin person with long thin legs