Sassing means talking back in a rude way or being disrespectful to someone in what you say.
Sassing is usually seen as impolite or disrespectful behavior.
Examples:
She was sassing her mom when she didn't want to do her homework.
Stop sassing me, I don't like it.
He was sassing the teacher and got in trouble.
The little boy was sassing his sister and making her mad.
I heard you sassing the principal, that's not good!
The cat was sassing the dog and it was funny.