While it may be that the religious streams now grouped together under the rubric of Hinduism are ancient, the word 'Hindu' was not applied to them until relatively recently.
The word “Hindu” is now taken to mean a person who follows what is called the Hindu religion, or Hinduism. It was not always so.
In Sanskrit (as in the earlier Indo-Aryan), “sindhu” means a large body of water and its usage is applied to rivers and oceans. The word was turned into the proper name of the largest river in the region, now called the Indus. The terms “Hind” and “al Hind” came to be applied to the Indian sub-continent – the region across the river – by Persians and Arabs starting around the 6th century BCE. The geographic name was applied to ethnicity and culture also. It had nothing to do with religion until much later.
Proponents of the “Hindu” religion, in particular those who follow the ideology of Hindutva, claim that it is the world’s oldest. While it may be that the religious streams now grouped together under the rubric of Hinduism are ancient, the word “Hindu” was not applied to them until relatively recently by those who followed these religious streams or religions. In DN Jha’s essay “Looking for a Hindu identity”, he writes: “No Indians described themselves as Hindus before the fourteenth century” and “Hinduism was a creation of the colonial period and cannot lay claim to any great antiquity”.
In the 18th century, the European merchants and colonists began to refer to the followers of Indian religions collectively as Hindus.