What is the origin of "Sindhu" river name?
From Sindhu we got India, Indus, Hindi and Hindhu, though some of there words have originated from Iranian cognate of Sindhu.
I was searching about this. Sindhu seem to have a clear Proto indo Iranian etomology but lack Proto Indo European origin. Tho some linguistics sometimes trace it to PIE "\*kiesdh". But it Is not well accepted.
Some alternate theories suggests it to be from Burushaski "sinda" which in burushaski is common word used for all rivers, even the Proto Indo Iranian word also means a large river.
The other other one lean towards it to be from Proto dravidian "\*cintu\*. Which means "date tree" Like that. So not well direct etomology
Some people also speculate it to be from BMAC substrate.
some say it to be from old indo aryan aka Sanskrit word "syand"-- "to flow" or "sedh" meaning "to hold back"