having no effect; waste of effort
Idiomatic expression likening a futile action to driving a nail into rice bran. Used to describe advice, criticism, or effort that produces no result.
Warning him is like driving a nail into rice bran—it has no effect.
Saying that is a waste of effort.
Standard kanji spelling for this idiomatic expression.
Mixed kana-kanji spelling; the first part is often written in kana for readability.
From the image of hammering a nail into rice bran (糠), which is too soft to hold it, symbolizing a pointless action.