underlip vs lip

underlip and lip both are nouns.

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

As nouns, lip is a hypernym of underlip; that is, lip is a word with a broader meaning than underlip:

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

underlip (noun)	lip (noun)
the lower lip	either the outer margin or the inner margin of the aperture of a gastropod's shell
	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