Hot on the heels of finishing my lunch, I've just read Nicola Twilley's excellent history of lunch. She writes:
As late as 1755, according to Samuel Johnson's definition, lunch was simply "as much food as one's hand can hold" - which, as Laura Shapiro, culinary historian and co-curator of the New York Public Library's new Lunch Hour NYC exhibition, recently explained to me, "means that it's still sort of a snack that you can have at any time of the day."
And it wasn't until later still - around 1850 - that lunch became a regular fixture between breakfast and dinner, added Rebecca Federman, the exhibition's co-curator, Culinary Collections Librarian at the NYPL, author of Cooked Books, and a star panelist at Foodprint NYC.
Her entry also includes a Q&A covering the technology of lunch, the definition of lunch and more. A brief preview from the entry's lunch technology discussion:
Before sliced bread, the lunch literature is full of advice on social distinctions and the thickness of bread in sandwiches. You slice it very thick and you leave the crusts on if you’re giving them to workers, but for ladies, it should be extremely, extremely thin. Women’s magazines actually published directions on how to get your bread slices thin enough for a ladies lunch. You butter the cut side of the loaf first, and then slice as close to the butter as you possibly can.
I recommend reading the whole thing - it's quite fun.