underweight vs boney

underweight and boney both are adjectives.

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

As adjectives, underweight and boney are synonyms defined as:

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

underweight (adjective)	boney (adjective)
being very thin	having bones especially many or prominent bones
	being very thin