Where does “武蔵野” come from?

武蔵野 (Japanese) comes from Japanese 野, from Middle Chinese 野.

Ancestry of “武蔵野”, step by step

Step	Language	Word	Meaning
1	Japanese	野	an area, field; the hidden part of a structure;...
2	Middle Chinese	野	—

武蔵野 — Etymology, Origin & Word History | Etymology Explorer