Bahun (बाहुन) is came from india early 18th Nepali Bhasi traditional Brahmins of Nepalese Pahari people, Indian Gorkhas, Lhotshampa Bhutanese and Burmese Gurkha peoples. Bahun is a term that is only applicable to Pahari-Brahmin's in Nepal.
References
Bahun Wikipedia(Text) CC BY-SA