(Toufu ni kasugai;
“A wood-clamp in tofu”)
Attempting something that’s guaranteed to produce no meaningful response or results. An utterly ineffectual action, like using a construction-strength fastener on an ultra-soft material like tofu. Like talking to a brick wall, or emptying the ocean with a thimble.
This very simple phrase begins with the noun 豆腐 (toufu), “tofu” – more literally “decayed beans,” or “fermented beans” – followed by the direction marker に (ni). “in(to).” And finally we get the noun 鎹 (kasugai), a metal fastener shaped like a large, shallow staple and used in carpentry.
A close synonym is 糠に釘 (nuka ni kugi), “a nail into rice bran.” Just make sure not to get your wires crossed; putting the nail into tofu or the kasugai into rice bran is considered an error. Similarly, you can compare and contrast this phrase with 暖簾に腕押し, which highlights the lack of response without making the same explicit judgment about whether the result is worthwhile. Just keep in mind not to arm-wrestle the tofu!
This is the to entry of the Kyoto iroha karuta set. It comes to us from an Edo-era Ninjoubon work titled 『仮名文章娘節用』 (Kana majiri musume setsuyou).
(“Ima no sakusen no mama ja donna ni doryoku shite mo toufu ni kasugai da yo. Shudan wo kaenai kagiri, kimi wa hitoshiai mo katenai mama da yo.”)
[“As long as you keep using this strategy, it doesn’t matter how much effort you put into it; it’s like nailing jello to a tree. You’re not going to a win a single match as long as you don’t change your methods.”]