back talk vs lip

back talk and lip both are nouns.

Word	Noun	Adjective	Verb	Adverb
back talk	Yes	No	No	No
lip	Yes	No	No	No

As nouns, back talk and lip are synonyms defined as:

Other synonyms of back talk include backtalk, mouth, sass, sassing.

back talk (noun)	lip (noun)
an impudent or insolent rejoinder	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