ὑπογάστρ-ιον · hypogastr-ion — LSJ
the lower belly from the navel downwards, the paunch, Hp. Aph. 4.80, Arist. HA 503a17, Sor. 2.48, etc.
the belly of a sea-fish, esp. of the tunny, a favourite dish at Athens, Stratt. 4, 31 (hex.), Ar. Fr. 364, etc.; whence the joke in Id. V. 195.