"sassing" in the noun sense
1. sass, sassing, backtalk, back talk, lip, mouth
an impudent or insolent rejoinder
"don't give me any of your sass"
"sassing" in the verb sense
1. sass
answer back in an impudent or insolent manner
"don't sass me!"
"The teacher punished the students who were sassing all morning"
Source: WordNet® (An amazing lexical database of English)
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