Green’s Dictionary of Slang

(US) cheek, ‘back-talking’.

1942		Pittsburgh Press (PA) 28 May 32/1: He shouldn’t have to take back-sass from anybody.
1968		(con. 1910) Kellner Aunt Selena 94: ‘Tom’s uncle gave him 10 quarts of oats. He said “Feed the white horse 2 quarts more than the red horse, and the black horse 2 quarts more than the white horse.” What do you think Tom should do?’ ‘I think he should hit his uncle over the head with the feed bucket, myself.’ Wilbur said this wasn’t arithmetic, it was back-sass [DARE].
2004		J. Bageant ‘Son of a Laboring God’ 🌐 Too much public education only gets working people riled up and full of backsass.