I came across an explanation of this in an article on JA a few years ago. Basically, dinner used to be about the middle of the day, but fashion was shifting it later and later back in the day (see a post by the ever-resourceful Julie W on this topic), so people were going hungry. In response to this, the habit arose of having something to eat somewhere in between breakfast and dinner, but there wasn't any word for it. Nuncheon appears to have only been used for a meal eaten while travelling (it's used in this sense in S&S). So we get all these mysterious references to cold meat, or spreading the table (see for example chapter 42 of Emma), at times that aren't dinner and aren't breakfast.

The word "lunch" in the sense of a midday meal appears to have shown up in the 1820s. I've included a link to an Online Eytmology Dictionary below.

So I'd guess that Charlotte was offering Lady Catherine something like what we'd call lunch, a meal a bit more filling than tea and cake. This passage in P&P gives the impression that the meal was a bit like a buffet - Lady Catherine could see the size of the joint of meat, so perhaps it was sliced as people felt hungry.