belly vs underpart

belly and underpart both are nouns.

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

As nouns, underpart is a hypernym of belly; that is, underpart is a word with a broader meaning than belly:

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

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

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