What is the poisoning the well fallacy?

Discrediting a person in advance so that whatever they go on to say is dismissed before it's even heard.

A pre-emptive form of ad hominem: unfavourable information about person A (true or not) is presented first, biasing the audience so they'll reject anything A subsequently claims.

"Before my opponent speaks, remember that she's been wrong about everything for years — so take whatever she's about to say with heavy scepticism."

The setup taints A's argument before A makes it, so the audience evaluates the person rather than the content. Even if the unfavourable information is accurate, it's not evidence against the claims A hasn't even stated yet.

Tip: The "well" is the audience's mind; poisoning it ahead of time is just ad hominem fired pre-emptively.