scrawny vs underweight

scrawny and underweight both are adjectives.

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

As adjectives, scrawny and underweight are synonyms defined as:

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

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