expression, noun
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.
彼に注意しても糠に釘だよ。
Warning him is like driving a nail into rice bran—it has no effect.
そんなことを言うのは糠に釘だ。
Saying that is a waste of effort.
Both mean a futile effort, but 糠に釘 emphasizes no effect from words or actions, while 暖簾に腕押し suggests pushing against something that yields no resistance.
Similar meaning of uselessness, but 豆腐にかすがい is less common and more literally about a clamp on tofu.
From the image of hammering a nail into rice bran (糠), which is too soft to hold it, symbolizing a pointless action.