And because it was expected that he would advance all mankind, he was, accordingly, called Prithu (the advancer). And because also he protected people from injuries (Kshatta), he was, therefore, styled of Kshatriya (protector from injuries).
प्रथयिष्यति वै लोकान् पृथुरित्येव शब्दितः। क्षताद्यो वै त्रायतीति स तस्मात्क्षत्रियः स्मृतः।।१३८