lip vs underlip

lip and underlip both are nouns.

Word	Noun	Adjective	Verb	Adverb
lip	Yes	No	No	No
underlip	Yes	No	No	No

As nouns, underlip is a hyponym of lip; that is, underlip is a word with a more specific, narrower meaning than lip:

lip: either of two fleshy folds of tissue that surround the mouth and play a role in speaking
underlip: the lower lip

Other hyponyms of lip include overlip.

lip (noun)	underlip (noun)
either the outer margin or the inner margin of the aperture of a gastropod's shell	the lower lip
the top edge of a vessel or other container
either of two fleshy folds of tissue that surround the mouth and play a role in speaking
an impudent or insolent rejoinder
(botany) either of the two parts of a bilabiate corolla or calyx