Poisoning the well is a preemptive logical fallacy where unfavorable information about someone is presented to an audience, with the intent of discrediting everything said by that person beforehand.

The origin of this phrase comes from the belief in medieval times that outbreaks of plague were caused by Jews poisoning the water supply. Suggesting that someone was not to be trusted after accusing them of the unrelated crime of poisoning the water was effective rhetoric, but bad logic.

Examples:

Before you listen to my opponent, may I remind you that he has been in jail.

Don't listen to what he says, he's a lawyer.

Poisoning the well is a special case of argumentum ad hominem.

See also: Guilt by association.

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