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).