Where does “初体験” come from?

初体験 (Japanese) comes from Japanese 初, from Middle Chinese 初.

初体験 (Japanese): first experience; first sexual experience

Definitions

Step	Language	Word	Meaning
1	Japanese	初	the first..., initial..., new.; a first, start, a...
2	Middle Chinese	初	—

初体験 — Etymology, Origin & Word History | Etymology Explorer