From 《礼记·学记》: '玉不琢,不成器;人不学,不知道' — 'jade uncarved makes no vessel; a person unschooled knows nothing.' The original image: raw material that fails to become a useful implement.
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Cutting. Most at home in parental scolding ('你这不成器的东西') and self-deprecation ('我这不成器的儿子').