underweight vs skinny

underweight is not a noun while skinny is a noun.

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

As adjectives, underweight and skinny are synonyms defined as:

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

underweight (noun)	skinny (noun)
	confidential information about a topic or person

underweight (adjective)	skinny (adjective)
being very thin	of or relating to or resembling skin
	giving or spending with reluctance
	being very thin
	fitting snugly