an action that has no effect; a futile attempt (the image is of trying to drive a nail into soft bran).
彼に注意しても糠に釘で、同じミスを繰り返す。
Even if you warn him, it's like driving a nail into bran—he repeats the same mistakes.
この資料で説得しようとしたが、相手には糠に釘だった。
I tried to persuade them with these materials, but it had no effect.
子どもに何度言っても糠に釘だと感じることがある。
Sometimes I feel that no matter how many times I tell the child, it's all in vain.