underbelly vs abdomen

underbelly and abdomen both are nouns.

Word	Noun	Adjective	Verb	Adverb
underbelly	Yes	No	No	No
abdomen	Yes	No	No	No

As nouns, abdomen is a hypernym of underbelly; that is, abdomen is a word with a broader meaning than underbelly:

underbelly: the soft belly or underside of an animal's body
abdomen: the region of the body of a vertebrate between the thorax and the pelvis

Other hypernyms of underbelly include belly, stomach, venter.

underbelly (noun)	abdomen (noun)
the quality of being weak or unprotected	the region of the body of a vertebrate between the thorax and the pelvis
the soft belly or underside of an animal's body	the cavity containing the major viscera; in mammals it is separated from the thorax by the diaphragm
lower side