noun
a person who provides information to the police or other authorities, often in exchange for special treatment or immunity
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The police used a stool pigeon to gather information about the criminal organization.
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I heard that Tommy is a stool pigeon for the boss, always telling him what we're up to.
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Don't trust Joey, he's a stool pigeon and will rat you out in a heartbeat.
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In the world of politics, it's common for politicians to use stool pigeons to leak information to the press.