Input Junkie

It's just assault in the sense of threatening someone with bodily harm, that's all. It might not qualify in a court of law if the person hearing the threat didn't take it seriously, but if you tell a stranger, "I'm going to hit you" -- as this dialogue seems to do, right?--and you look like you actually might (which maybe isn't happening here) that's assault.

I googled to verify my impression that threatening violence was the legal definition of assault, and Professor Google spits this out--

Generally, the essential elements of assault consist of an act intended to cause an apprehension of harmful or offensive contact that causes apprehension of such contact in the victim.

The act required for an assault must be overt. Although words alone are insufficient, they might create an assault when coupled with some action that indicates the ability to carry out the threat. A mere threat to harm is not an assault; however, a threat combined with a raised fist might be sufficient if it causes a reasonable apprehension of harm in the victim.

So yeah, I don't think I would tell a stranger that I planned to spank her.