Where does “予備役” come from?

予備役 (Japanese) comes from Japanese 役, from Middle Chinese 役.

予備役 (Japanese): reserve service

Definitions

  1. reserve service

Ancestry of “予備役”, step by step

予備役 traces back through two separate lines of ancestry.

via Japanese

StepLanguageWordMeaning
1Japaneserole, service, use; hand, scoring combination;...
2Middle Chinese

via Japanese 予備

StepLanguageWordMeaning
1Japanese予備reserve (that which is reserved, or kept back, as for future use), spare, preliminaries
Every word from Middle Chinese Every word from Japanese