Where does “私書箱” come from?

私書箱 (Japanese) comes from Japanese 箱 — box.

Ancestry of “私書箱”, step by step

私書箱 traces back through two separate lines of ancestry.

via Japanese

StepLanguageWordMeaning
1Japanesebox

via Japanese 私書

StepLanguageWordMeaning
1Japanese私書private document

Words derived from “私書箱

Every word from Japanese
私書箱 — Etymology, Origin & Word History | Etymology Explorer