I have found it odd that the term "aryan" and swastika were adopted by the Nazis. Aryan has been a term used to describe Indo-Iranaian people, not the blonde hair, blue eye übermensch. The Swastika has been used across Asia and is still used in Indian religions. Why did they appropriate these from cultures so far away, and clearly not Germanic in origin, to tout Germanic superiority? Did Hitler or some other ranking official go on some Beatles-esque retreat to India?
The Germans were simply relying on the work of scholars such as Max Muller who had advanced the field of Comparative Linguistics (then called Philology) and Indo European studies, building on the work of people such as William Jones.
Basically, Jones had shown that Sanskrit and Greek had a lot of commonalities in language and must have come from a common root. This evolved to include a bunch of other languages which today are part of the Info European family. At the time, it had given rise to the idea of a common root culture, who became called the Aryans. The Nazis (and other groups) had appropriated the question of where this community originated to make it about who the "true" Aryans were, and came to the conclusion it was them, based on ideas of racial superiority and then attitudes also prevalent in fields such as Anthropology.
It is worth noting one thing - it's extremely problematic to say that Aryan is a term that originated in India. We don't quite know where the term originated. It was used in both Ancient India and Ancient Persia (they had a province called Ariana in Greek renderings) and indeed the very word Iran derives from Arya. The western branches of the Indo European languages don't appear to posses the word Arya or other derivates, but other words to exist. Divine and Deos for instance are related to the Indian and Persian Deva and Daeva. Similarly Pater, Father and Pitr.