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."
Someone who is being a smart ass and sarcastic. Used in either (1) combination or (2) consecutively- such as using a smart ass remark and then using a sarcastic remark immediately there after. Can be used vice versa
Person a: Are you painting?
Person b: Yeah. I have a brush in my hand, paint on the brush.
Person a: God you are so freaking smartastic.
Person a: Is that a speed trap? (the cops stops traffic)
Personb: Oh, yeah because a cop would put a barricade up if they were trying to catch you speeding.
Person a: Why do you have to be so smartastic to me all the time?