back talk vs sassing

back talk and sassing both are nouns.

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

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

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

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