The word Hindu came from an old Persian word. In the 6-5 centuries BC, this word was used to indicate the region lying east of the River Sindhu.
The Arabs continued using this expression. They called this region Al-Hind and the people ‘Hindi’. Later, the Turks started calling the people Hindu and their land as Hindustan and their language Hindavi. None of these was indicative of any religion. The word Hindu was used to indicate somebody’s religion much later.