Poisoning the well
Etymology
The metaphor recalls the old atrocity, and old libel, of poisoning an enemy’s water supply. John Henry Newman gave it its rhetorical sense in the 1860s, accusing his critic Charles Kingsley of poisoning the well against him before the argument began. The image fits: contaminate the source and no one will trust what flows from it.
noun phrase · /POY-zuh-ning thuh WEL/
Discrediting a source in advance so that anything they later say is dismissed.
What it names
Spotting it protects a speaker’s actual argument from being buried under a preemptive smear.
Example
"Before she speaks, remember she’s been wrong before" primes the room to ignore her — poisoning the well.
Related idea
It is a preemptive ad hominem: the attack lands before the argument is even made.