Sass is a smart alecky or impudent reply. Your middle school teacher might warn you that once you get to high school, they won't put up with your sass.
Use sass as a noun — or a verb, meaning to talk in a cheeky way: "Don't you dare sass your grandmother!" When someone teases you or speaks in an almost rude manner, they give you sass. This word emerged in the 19th century from the adjective sassy, which began as a variation on saucy, from the idea that words can be zesty, sharp, or spicy.