boney vs underweight

boney and underweight both are adjectives.

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

As adjectives, boney and underweight are synonyms defined as:

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

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