From the Latin oestrus ("gadfly”, “sting”, “frenzy"), from the Ancient Greek οἶστρος (oistros).
From Ancient Greek prefix ἱερo- (hiero-), from ἱερός (hieros, "sacred, holy")
From Proto-Indo-European *ish₂ros. There are a number of candidate cognates with this word. Compare Sanskrit इषिर (iṣirá) and Oscan 𐌀𐌉𐌔𐌖𐌔𐌉𐌔 (aisusis).
Both the two words derive from the same PIE noun *ish₂ros, but why did the Ancient Greek decendants own different meanings with a prefixed "h" from each other, further giving "hiero-" and "oestrus"?