Where does “青菜に塩” come from?

青菜に塩 (Japanese) comes from Japanese に, from Old Japanese に.

Ancestry of “青菜に塩”, step by step

青菜に塩 traces back through two separate lines of ancestry.

via Japanese に

StepLanguageWordMeaning
1Japaneseにparticle for indirect objects; to; particle for...
2Old Japaneseに—

via Japanese 塩

StepLanguageWordMeaning
1Japanese塩salt; saltiness; hardship, toil, trouble
2Middle Chinese鹽—
Every word from Old Japanese に →Every word from Japanese に →