Poisoning the well
Poisoning the well (or attempting to poison the well) is a type of informal logical fallacy where irrelevant adverse information about a target is preemptively presented to an audience, with the intention of discrediting or ridiculing something that the target person is about to say. Poisoning the well can be a special case of argumentum ad hominem, and the term was first used with this sense by John Henry Newman in his work Apologia Pro Vita Sua (1864). The origin of the term lies in well poisoning, an ancient wartime practice of pouring poison into sources of fresh water before an invading army, to diminish the attacking army's strength.
A poisoned-well "argument" has the following form:
- 1. Unfavorable information (be it true or false) about person A is presented by another. (e.g. "Before you listen to my opponent, may I remind you that he has been in jail")
- 2. Therefore, the claims made by person A will be false. 
A subcategory of this form is the false dilemma; an unfavorable attribute to any future opponents, in an attempt to discourage debate. (For example, "That's my stance on funding the public education system, and anyone who disagrees with me hates children.") Any person who steps forward to dispute the claim will then risk applying the tag to him or herself in the process.
A poisoned-well "argument" can also be in this form:
- 1. Unfavorable definitions (be it true or false) which prevent disagreement (or enforce affirmative position)
- 2. Any claims without first agreeing with above definitions are automatically dismissed.
- Appeal to ridicule
- Black propaganda
- Framing (social sciences)
- Fruit of the poisonous tree
- Guilt by association
- Scorched earth (military doctrine involving literal well poisoning)