If you're going to be a smart ass, you have to be smart, or you're just an ass.
Smart Ass Definition
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Explanation
This is a play on words. The phrase 'smart ass' implies someone who is both clever and annoying. The joke suggests that if someone is trying to be a 'smart ass' but lacks the 'smart' part, they just end up being an 'ass', which is just a jerk.
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