Source : Webster's Revised Unabridged Dictionary (1913)

Underpart \Un"der*part`\, n.
   A subordinate part.

         It should be lightened with underparts of mirth.
                                                  --Dryden.

Source : WordNet®

underpart
     n : a part lying on the lower side or underneath an animal's
         body; "the warbler has a white throat and underparts"; "a
         woodland mouse with white underparts"