"Saptasindhu as the name of the ancient region of the Seven Rivers in N-W India and Pakistan - countries which did not exist at that period. I use it as a bahuvrīhi, as many others have done before me, although in the RV we find references only to the Seven Rivers saptá síndhavaḥ (and different oblique cases of the plural). Now (e6) Avestan has the name Haptahǝndu as a place, like Airyana Vaējah, Raŋhā, Haetumant, etc, from which the Iranians had passed before settling down in eastern Iran, then spreading west and north. But what is this name? Yes, hapta- is the numeral ‘seven’ but what of hǝndhu? It is a fairly obvious Avestan correspondence to the Sanskrit síndhu. Now hǝndu is an isolated occurrence. The stem does not otherwise exist in Avestan. Hindu appears in Old Persian indicating the Indian province under the Achaemenids, and that is all. The interpretation ‘seven rivers’ comes from the Sanskrit collocation. But the Avestan for river is usually θraotah- (=S srotas) and raodah-.... Surely nobody would be so foolhardy as to suggest that the IAs took this otherwise unattested stem from Iranian and used it so commonly and productively."