Poisoning the well is a logical fallacy where adverse information about a target is preemptively presented to an audience, with the intention of discrediting or ridiculing everything that the target person is about to say. Poisoning the well is a special case of argumentum ad hominem.
The person making such an attack is hoping that the unfavorable information will bias listeners against the person in question and hence that they will reject any claims he might make. However, merely presenting unfavorable information about a person (even if it is true) hardly counts as evidence against the claims he/she might make.
1. Unfavorable information (true or false) about person A is presented by another.
2. Any claims person A then makes will be regarded as false or taken less seriously.
1. “That's my stance on the public education system, and anyone who disagrees with me hates children.”
2. “Before you listen to my opponent, may I remind you that he has been in jail”