Definition of belly-up:
(n) :
The abdomen (especially a fat one).
(n) :
stomach (an organ in animals that stores food in the process of digestion)
(n) :
(anatomy, countable) uterus (a reproductive organ of therian mammals in which the young are conceived and develop until birth)
(v) :
(intransitive) To swell and become protuberant; to bulge or billow.
(v) :
(transitive) To cause to swell out; to fill.
(n) :
The part of anything which resembles (either closely or abstractly) the human belly in protuberance or in concavity; often, the fundus (innermost part).