Where does “რუსეთი” come from?

რუსეთი (Georgian) comes from Georgian -ეთი, from Proto-Georgian-Zan *-et.

Ancestry of “რუსეთი”, step by step

Step	Language	Word	Meaning
1	Georgian	-ეთი	location suffix; geographical suffix
2	Proto-Georgian-Zan	*-et	—

რუსეთი — Etymology, Origin & Word History | Etymology Explorer