IPA: /ˈʌndərpɑːrt/
KK: /ˈʌndərˌpɑrt/
A section or part that is located underneath or on the lower side of something, often referring to the underside of an animal or object.
The underpart of the bird was covered in soft feathers.
Underpart → It is formed from "under" (from Old English "under", meaning beneath or below) and "part" (from Latin "partem", meaning a portion or piece). The word "underpart" refers to a portion that is beneath or lower than another part.
Think of something that is 'beneath' ('under') another 'piece' ('part') — that's why underpart means the lower portion.