Where does “онсэн” come from?

онсэн (Russian) comes from Japanese 温泉, from Middle Chinese 溫泉 — hot spring.

онсэн (Russian): onsen

Definitions

Step	Language	Word	Meaning
1	Japanese	温泉	a hot spring; a spa, an onsen
2	Middle Chinese	溫泉	hot spring

онсэн — Etymology, Origin & Word History | Etymology Explorer