When it comes to fixing cars, I'm a complete amateur.
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culture
Before washing machines, women across China beat clothes with a 棒槌 on a flat stone or board (捣衣 dǎoyī). The image is so iconic that 'beating clothes by moonlight' became a poetic motif — see Li Bai's 《子夜吴歌》.
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The 'clueless amateur' meaning is very colloquial Northern slang — calling someone a 棒槌 is a teasing dig at their incompetence in something specific. Don't use in formal speech.