In
the Shahnameh, Rostam is referred to as Sagzī (= Arabicized: Sejzī),
which is derived from the name Sīstān. Previously, this region was known
as Sakistān/Sagistān, a name derived from the Iranian-descent Saka
people. The first part of the name, Sak, comes from this origin. Hence,
the older form Sakzī is the original, which later transformed into Sagzī
due to the change of k > g [1]. As for -zi, It is a locative suffix found in various regions throughout Iran.
For
-zi and other locative suffixes such as -chi and -ji in
Persian, and examples used in place-names, see the article by Marquart:
Marquart 1895, “Beiträge zur Geschichte und Sage von Ērān”, ZDMG 49: 664sqq.
This
-zi is also found as -zzi in Hittite. Refer to Benveniste's book:
Benveniste 1962, Hittite et indo-européen: 102-105.
[1] For the phonetic transformation of k > g, see Gray 1902, Indo-Iranian Phonology: §116.
