That’s not an easy question to answer in a historical sense. So many things have gone by the name “shortcake” over the centuries that it’s become impossible to trace the word to a single point of origin. The term famously appears in Shakespeare, in The Merry Wives of Windsor, however since the Bard failed to include a recipe with the stage notes, we have scant idea what sort of cake he actually meant.
A short cake, for most of the last several hundred years, has been pretty much what it sounds like: a not-very-tall oven- or griddle-baked thing — possibly sweetened, possibly not — made with a grain flour of some sort, perhaps wheat, maybe barley or oats. The thing that makes a shortcake “short”, i.e. that which keeps it from rising terribly much, is fat. Fat keeps short bread from attaining much height as well, hence the name. No wonder that fat all by itself is also known as “shortening.”
So a shortcake, at least classically, was very a very broadly defined thing: a fat-enriched grain cake. How did we get from there to the modern American shortcake which is an extremely narrowly-defined thing made with (American) biscuits, strawberries and whipped cream? Unknown. What we do know is that very abruptly, around the year 1850, strawberry shortcake became all the rage among polite society folks in the Eastern US. Women’s tea groups suddenly started holding “shortcake parties”, and recipes began to proliferate.
The first of these, from Miss Leslie’s Ladies Receipt Book, was published in 1847 and called for a dense, buttery biscuit dough, cut into rounds and stacked with mashed, sweetened strawberries as a filling. To finish, a thick icing was applied to the top. The recipe quickly evolved to become a showcase for not only fresh, seasonal strawberries, but fresh whipped cream as well. Today, while star chef and cookbook authors forever taking riffs on the original recipe, the best versions remain faithful to the first: simple, sweet biscuit sandwiches piled with fresh fruit.