How does Gk. "hieros" differ from its cognate Gk. "oistros" with an "h"?

Question

oestrus

From the Latin oestrus ("gadfly”, “sting”, “frenzy"), from the Ancient Greek οἶστρος (oistros).

hiero-

From Ancient Greek prefix ἱερo- (hiero-), from ἱερός (hieros, "sacred, holy")

ἱερός

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"?

Answer 1

I'll try to give a partial answer.

According to Chantraine (Dictionnaire Étymologique Grec), Ancient Greek oistros and hieros might be related but this is not sure.

And Ancient Greek hieros does not have a clear well-defined meaning but several ones, so some linguists believe that two formerly separate words have been merged into one word (hieros).

Returning to your question: "... why did the Ancient Greek decendants own different meanings ..." — Maybe their meanings are so different from each other because they are not related. Sure, there are drastic semantic changes but if meanings are too different, a theory of relationship becomes too weak.

Answer 2

Mine is also a partial answer. I'm not commenting on the etymology of the Greek words in your question.

In Attic Greek ἱερός, there's a diacritic over iota (in some other dialects there was psilosis, or h-dropping). It's called "rough breathing" (Lat. spiritus asper; Greek δασὺ πνεῦμα). Usually, Anlaut (word-initial) spiritus asper in Greek corresponds to PIE *s.

In the case of Greek ἱερός, it's a different story. Sihler 1995 argues that

"Initial /h/ sometimes results from the anticipation of an intervocalic /h/ from -s-. (§174.2)"

There's also another rule here: an intervocalic *s was lost in Greek (*s>*h>0) (Sihler 1995, §172).