The story
From सप्त (sapta, “seven”) + सिन्धु (síndhu, “river, stream”). This expression in English incorporates a grammatical error: Sindhu is the nominative singular; used with "seven", the noun should, of course, be in the plural: Sindhava:/Sindhavas, सप्तसिंधव: (saptasiṃdhava:, “seven rivers”), which is the term used in Sanskrit.
Etymology from Wiktionary, CC BY-SA 4.0.