From "Sapiens: A Brief History of Humankind"

After William Jones argued that all Indo-European languages descend from a single ancient language many scholars were eager to discover who the speakers of that language had been. The earliest Sanskrit speakers who had invaded India from Central Asia more than 3000 years ago had called themselves Arya. The speakers of the earliest Persian language called themselves Airiia. European scholars consequently surmised that the people who spoke the primordial language that gave birth to both Sanskrit, Persian, Greek, Latin, etc. must have called themselves Aryans.

Next European scholars wedded the linguistic theory about the industrious Aryans to the theory of natural selection and posited that the Aryans were not just a linguistic group but a biological entity - a race. And not just any race but a master race of tall, light-haired, blue eyed, hard working superhumans who emerged from the north to lay the foundations of culture throughout the world. Regrettably the Aryans who invaded India and Persia intermarried with the local natives they found in those lands, losing their light complexions and blond hair, and with them their rationality and diligence. The civilizations of Persia and India consequently declined. In Europe on the other hand, the Aryans preserved their racial purity. This is why Europeans had managed to conquer the world, and why they were fit to rule it." p.337-338

The nazi idea of "the Aryan race" was not so much a race classification but rather a measure of "fitness". We hear a lot about the blond hair blue eyed man as being the Nazi ideal but that is an over simplification of the issue. Hitler believed that the German race was becoming feminized and the way he countered that was by creating a new ideal "the aryan man" as the standard to live up to. The politics of nazi Germany and even Hitler himself was more concerned about gender roles in regards to the "fitness" of its citizenship. While there weren't very many blonde hair blue eyed nazis the point of the "aryan" prestige was to illicit a sort of group ideal. It served as a way to get the citizens to conform to a mass ideology that would promote German exceptionalism. Hitler and the nazi party were reactionaries. They believed that to get Germany out of its post WWI demise they needed to weed out any weak links in the citizenship and to do that they raised the "standards" of citizenship.

If you care to read more about where I got this info check out "gender, sex and the shaping of modern Europe" by Timm and Sandborn. There's a whole chapter called "brothers and sisters at war" that gives a pretty good survey on Nazi ideals and what the Aryan race meant to the Reich.