Washing sediment in a pan to search for valuable minerals like gold; also means criticizing someone or something harshly.
The gold-panning sense comes from using a pan to separate heavy gold from lighter sediment. The criticism sense appeared in the 1880s, possibly from the idea of 'washing away' the good qualities and finding only bad ones.
Gold miners in the 1800s used the same 'panning' technique that had been used for centuries—but movie critics borrowed the word in the 1880s because they were also 'sifting through' films to separate out what was worthless and bad!
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