That’s a tough question, reader Janey. You’re quite correct that the word “brioche” has existed in the French language since about the year 1400. It’s a variant of the Old Norman broyer which means “to knead” or “to break up.” However just because the word has existed for that long it doesn’t mean that anything like modern brioche was around back then.
What did exist in Normandy in those days was bread, but also butter. Indeed Normandy has been producing some of the world’s best butter for about 500 years. Did they combine the two? There’s no way to know for sure, but let’s assume they did. The result would have been something on the order of a dense whole grain boule with butter incorporated into it. After all the Normans were rural folk, and wouldn’t have access to finely milled, finely bolted white flours, which didn’t come along until about the mid-1700’s in France anyway. They also would have used slow-rising natural levains (starters) which can’t deliver the light, fluffy textures that packaged yeast cultures can.
So my thinking on this is that it was the Viennese who brought what we now know as brioche to France, specifically Paris. They had the technology, as the Six MIllion Dollar Man fans of old like to say. They had the flour, the leavening, the ovens…and don’t discount the inclination to mix bread dough with lots of very rich additives.
The French probably just adapted the old word to fit the new imported product. To be sure, brioche got even better when Norman butter was added to it. So it’s not as though brioche isn’t French, it’s just that it’s not solely French, and that’s OK I think.