noun
feeble anger; impotent rage
Idiomatic expression for anger or frustration that one cannot act on because one is powerless. Often used self-deprecatingly or to describe a weak person's futile resistance. Literally 'the clenching teeth of a dried sardine'.
彼の抗議など、ごまめの歯軋りに過ぎない。
His protest is nothing more than the clenching teeth of a dried sardine.
小さな会社が大企業に文句を言っても、ごまめの歯軋りだ。
Even if a small company complains to a big corporation, it's just impotent rage.
歯軋り alone means 'grinding one's teeth' (in anger or sleep). ごまめの歯軋り is a set phrase that adds the image of a tiny dried fish to emphasize powerlessness.
From ごまめ (dried sardine) + の + 歯軋り (teeth grinding). The image is of a tiny, dried fish grinding its teeth in anger, which is futile and laughable. The exact origin is uncertain, but it is a traditional Japanese idiom.