Interpretation of Sanskrit Indra as cognate of Slavic je,dru "swift, impetuous" points to a PIE *yendro-, yNdro. Greek hadros "strong, thick" too?
But...
The most correct Skt. form should be *Yandra, *Yadra, or not? May years ago, I've ever thought in relate Indra to Sumerian Ninurta (Enurta) or Egyptian Anhert (Onuris), or even Greek Ares, but I confess that these bettings are not very plausible.
JS Lopes
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