underweight vs scrawny

underweight and scrawny both are adjectives.

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

As adjectives, underweight and scrawny are synonyms defined as:

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

underweight (adjective)	scrawny (adjective)
being very thin	inferior in size or quality
	being very thin