In the earliest Indian text (the Rig Veda dating back several millennia) the word jan refers to a tribe/people while the ruler or protector of that tribe is called the Ra-jan. The Rajan's wife is called the Rajani and his brothers, cousins, and nephews are called Rajanyas (also pronounced Rajanka). These Rajanyas provided the armed strength to protect the jan while the Rajan was their leader in war and an administrator/judge in times of peace.
In later times Rajan evolved into Raja (King), Rajani into Rani (Queen), and Rajanya into Rana (chieftain). Such changes were of course more immediate in the spoken languages like Prakrit or Apabhramsa and slower in the classical language of Sanskrit. As an illustration, the Pratihar clan have recorded an inscription near modern Jodhpur (dated 837 CE), which refers to their queen as the Maharajni (great queen)…this rajni would be an Apabhramsa word from the original Sanskrit rajani. A second example, this time from the east, will further illustrate this process of word evolution. The Bhanja dynasty in Orissa issued copper-plate grants in the 9th Century that mention princes with the title of Ranak…again an Apabhramsa word between the Rajanka[1] of ancient times and the Rana of a later age[2].
As populations grew and tribal territories expanded into monarchies, the Rajanyas must have become landowners/rulers in their own right with the word Rana becoming a hereditary title. And when these Rajanyas became chieftains another word was needed for the mass of ordinary soldiers and nobles who were not chiefs—this new word was Kshatriya or warrior[3]. It is in this later Vedic age that we read for the first time of the word Rajaputra (the Raja's son).
This is the right place to mention the historical personalities associated with the word Rajaputra. The famous founder of Buddhism, Siddharth, was a prince who left his family to contemplate the meaning of life. Since he never returned to become king after his father's death, he was always called Rajaputra Siddharth in all Buddhist texts. This was in the 7th Century BCE—more than eighteen centuries before the Turk-Rajput wars!
The second historic personality is Harshvardhan, the younger brother of Rajyavardhan, in the Kingdom of Thanesar. The elder brother became ruler of Kannauj in the 7th Century CE while Harshvardhan, as the junior prince, was called the Rajaputra Siladitya. Even after becoming king Harsh continued with the designation of Rajaputra until his position on the throne was secure...all this happened 600 years before the Turk-Rajput wars.
The break-up of the Maurya Empire in northwest India and the intrusion of foreign powers like the Yavans, Sakas, and Kushans, led to some interesting changes in the use of titles, which is described in this post.