Neck or Nuts?

Here is a Jaidynism before I forget.

On the way home from church today, Tom was being Tom and driving a bit crazy, mostly just to annoy me and make the girls laugh. From the back seat we hear a sigh and a groan. Next, we hear, "Dad, what are you doing? You are such a pain in the nuts!"

I laugh and try and tell her that it is not "nuts" but "neck". She then told me that "neck" does not make any sense, she will just say "nuts". I was laughing too hard to explain any further.