underpart vs belly

underpart and belly both are nouns.

Word	Noun	Adjective	Verb	Adverb
underpart	Yes	No	No	No
belly	Yes	No	Yes	No

As nouns, belly is a hyponym of underpart; that is, belly is a word with a more specific, narrower meaning than underpart:

underpart: a part lying on the lower side or underneath an animal's body
belly: the underpart of the body of certain vertebrates such as snakes or fish

underpart (noun)	belly (noun)
a part lying on the lower side or underneath an animal's body	the underpart of the body of certain vertebrates such as snakes or fish
	the region of the body of a vertebrate between the thorax and the pelvis
	a protruding abdomen
	the hollow inside of something
	a part that bulges deeply

underpart (verb)	belly (verb)
	swell out or bulge out