A person or thing that is mocked or ridiculed by many people; an object of ridicule.
From 'laughing' + 'stock' (a person or thing considered as something to be exploited). The term dates to the 1600s, combining the idea of laughter directed at someone with 'stock' meaning a target or object of treatment.
Stock originally meant a wooden post or pole—a target where laughter was directed. It's a vivid medieval image of someone as a fixed target for public mockery, like someone in stocks (the punishment device)!
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