Someone who is a smart ass or sarcastic. If you're around people you're hesitant to say the word ass in front of, yet want to call someone a smart ass, this is the perfect solution.
Sally: "Nothing is worse than someone waking you up from an awesome dream!"
Jesse: "Really? Not even death, cancer, a car wreck, or a billion other things?"
Sally: "Don't be smartastic, you know what I'm saying."