underweight vs lean

underweight is not a noun while lean is a noun.

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

underweight (noun)	lean (noun)
	the property possessed by a line or surface that departs from the vertical

underweight (adjective)	lean (adjective)
being very thin	lacking in mineral content or combustible material
	lacking excess flesh
	not profitable or prosperous
	containing little excess

underweight (verb)	lean (verb)
	rely on for support
	cause to lean or incline
	to incline or bend from a vertical position
	cause to lean to the side
	have a tendency or disposition to do or be something; be inclined