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The synonym of backtalk is Tattle sassing
lip
The word is lip. It means to talk back in a rude manor.
Most dictionaries have back talk listed as two words in American English, but I also found backtalk, one word in some online dictionaries.
To be safe, I would write as two words.
sasalthough i don't think that it is a proper word.
'Lip' is the word. Sass has four letters.