underweight vs bony

underweight and bony both are adjectives.

Word	Noun	Adjective	Verb	Adverb
underweight	No	Yes	No	No
bony	No	Yes	No	No

As adjectives, underweight and bony are synonyms defined as:

Other synonyms of underweight include boney, scraggly, scraggy, scrawny, skinny, weedy.

underweight (adjective)	bony (adjective)
being very thin	having bones especially many or prominent bones
	composed of or containing bone
	being very thin