This comes from the myth around Hindu word where Arabs used it with dishonoured meaning @RShivshankar
While, truth is that the word “Hindu” was first used by Darius-1, in 6th cen BC, for the people who lived beyond Sindhu.
Hindu comes from Sindhu. It is absolutely perfect.

The Sapta Sindhu is referred as Hapta Hindu in Zend Avesta.
Moreover if one follows the latest discoveries around the subject of migration @RShivshankar , it becomes clear that the Persians were the people who migrated out of India after battle of the kings (rig Veda).

One can refer to works of Talegeri ji which explains the whole thing very well and has even rebutted Tony Joseph which became one major tool to impose Aryan being a race who migrated into Bharat.
Tony Joseph couldn’t reply back at all except for rhetorics.

And Darius in his inscription too considers himself Ārya, which further reinforces vaidik heritage identity to the word Hindu.
The Persians have been mentioned as Parsu tribe in RigVeda.

This becomes even more concrete from the fact that Darius-1 calls his home Parsa, while older Assyrian inscription from 844 BC, uses term “Parshu”which matches with Rigvaidik term of tribe Parsu.