Orgination and Derivatin of the Tharu

Background
In 1978, When I was 11 years old, once i quationsed my grandfather why we call ourselves Tharu. My grandfather was not a well educated person but a mere farner, so he could not reply directly. Instead, he told me a dramatical rale about my great grandfather. His name was Buddhasen 'Patawari' (BS 1927-2012) He was regarded as a well known 'Vidwan' of Sanskrit and shanker Vedant in the Local region.