skinny vs underweight

skinny is a noun but underweight is not a noun.

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

As adjectives, skinny and underweight are synonyms defined as:

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

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

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