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A Judas goat was an animal, such as a goat, used in stockyards to lead other sheep to slaughter, while the Judas goat itself was allowed to escape at the last moment. When applied to Humans, the term refers to one who leads someone else into trouble, and manages to keep themselves free of harm.
When convinced by James T. Kirk and Spock to draw the Companion out into the open so that it could be neutralized, Zefram Cochrane recalled the term, as he regretfully knew that he was leading the Companion into a trap. (TOS: "Metamorphosis")
The term comes from the Biblical character Judas who betrayed Jesus.