RAJBUNSI or Rajbansi is the modern name of a people of Kocch-Behar, originally belonging to the great Kocch tribe, from which Kocch—commonly called Kocch-Behar—took its name. Their territory originally extended from 88° to 93° E. long., and from 25° to 27° N. lat., and in union with the Mecch or Bodo tribe, they were long successful in keeping out the invading Moslems, Bhootanese, and Assamese. But the grandson of the chief who originated this policy of union, cast off the Bodos, and, with all the people of condition, apostatized to Hinduism : “ the country was renamed Bihar, the people Rajbansis, or descendants of princes,” (see Journal Asiatic Society, Bengal, vol. vii., p. 12), so that only the lowest of the people “ could tolerate the very name of Kocch, and most of these being refused a decent status under the Hindu regime, yet infected with the disposition to change, adopted Islam in preference to helot Hinduism. Thus the mass of the Kocch people became Mahomedans, and the higher grades Hindus; both style themselves Rajbansis ; a remnant only endure the name of Kocch, and of these but a portion adheres to the language, creed, and customs of their forefathers.”
From the Book "The people of India : a series of photographic illustrations, with descriptive letterpress, of the races and tribes of Hindustan"