Where does “室内楽” come from?

室内楽 (Japanese) comes from Japanese 楽, from Middle Chinese 樂.

室内楽 (Japanese): chamber music

Definitions

Step	Language	Word	Meaning
1	Japanese	楽	comfortable, enjoying; easy, simple; rich,...
2	Middle Chinese	樂	—

室内楽 — Etymology, Origin & Word History | Etymology Explorer