You call someone a "smart ass" in a very specific situation:
- You say something that's not supposed to be taken seriously. For example, you exaggerate something or speak sarcastically.
- The other person knows that you're not being serious.
- However, they act like you were being serious anyway.
In this situation, you can say that they're "being a smart ass". Here are some examples:
A: I love this restaurant!
B: You love it? Why don't you marry it then?
A: Don't be a smart ass!
A: I'm so hungry I could eat a horse!
B: Oh really? Do you want me to butcher one for you?
A: Whatever, smart ass!
You can also call someone a "smart Aleck". The meaning is the same, but it's a little more polite.