Poisoning the well is a logical fallacy where adverse information about someone is preemptively presented to an audience, with the intention of discrediting or ridiculing everything that person is about to say.
This sort of "reasoning" involves trying to discredit what a person might later claim by presenting unfavorable information (be it true or false) about the person. This "argument" has the following form:
Unfavorable information (be it true or false) about person A is presented.
This sort of "reasoning" is obviously fallacious. 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. This is
especially clear when Poisoning the Well is looked at as a form of ad Homimem in
which the attack is made prior to the person even making the claim or claims.
"Don't listen to him, he's a scoundrel.""Before turning the floor over to my opponent, I ask you to remember that those who oppose my plans do not have the best wishes of the university at heart."
You are told, prior to meeting him, that your friend's boyfriend is a decadent wastrel. When you meet him, everything you hear him say is tainted.