underweight vs weedy

underweight and weedy both are adjectives.

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

As adjectives, underweight and weedy are synonyms defined as:

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

underweight (adjective)	weedy (adjective)
being very thin	abounding with or resembling weeds
	being very thin