Underpart n. : A subordinate part.

Example:
How do you whiten the Underpart of your nails?

Taxonomy
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Defination from the web:
underpart in the Online Dictionary. Meaning of underpart. Pronunciation of underpart. Translations of underpart. underpart synonyms, underpart antonyms. ... underpart - a part lying on the lower side or underneath an animal's body; "the warbler has a white throat and ...