Unfortunately, this is needs to be said, because some people still think this is OK.
Never smack a woman on the ass without prior consent.
It shows a complete disregard for all women, not just the woman you are violating. It shows that you feel that you are entitled to women’s bodies. It shows that you know you have the societal power to get away with it. It shows women exactly what you think of them, and what their place should be, far under yours. This is the same brand of thinking that enables rape.
So stop that shit unless you really are a misogynist. Don’t even joke about doing it.