There is a cat shut inside.
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The English translation of the Spanish hay gato encerrado, which is in turn the Spanish idiom for the Pig In a Poke trick, a scheme popular in the middle ages, “when meat was scare and cats were not.”
It “entailed the sale of a suckling pig in a poke (bag). The wriggling bag would actually contain a cat (not particularly prized as a source of meat) that was sold to the victim in an unopened bag.”