'Underpart' definitions:

Definition of 'underpart'

(from WordNet)
noun
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"

Definition of 'Underpart'

From: GCIDE
  • Underpart \Un"der*part`\, n. A subordinate part. [1913 Webster]
  • It should be lightened with underparts of mirth. --Dryden. [1913 Webster]