Bié mà tā dòuzhā nǎojīn, tā zhǐshì méi rènzhēn tīng.
Don't call him a porridge head - he just wasn't paying attention.
Tips
memory
豆渣 is the gritty, useless leftover pulp from making tofu - soft and shapeless. Calling someone 豆渣脑筋 ('tofu-pulp brain') says their thinking is just as mushy and worthless.
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Crude colloquial insult. Mild compared to true profanity, but still rude - keep it for joking among close friends.
