Godwin’s law
Etymology
Coined by attorney Mike Godwin in 1990, originally aimed at Usenet discussions, partly as a deliberate memetic experiment to make people think twice before reaching for Nazi comparisons. Over time a corollary emerged: whoever first invokes the comparison is often deemed to have lost the argument. Godwin has clarified that the law is not meant to shut down apt historical comparisons, only lazy ones.
noun phrase · /GOD-winz law/
The observation that as an online discussion grows longer, the probability of a comparison to Hitler or the Nazis approaches certainty.
What it names
It captures a real dynamic of prolonged argument — the slide toward the most extreme available analogy — and gives people a way to call it out.
Example
The thread was civil for a day, then Godwin’s law arrived and someone called the parking rule literally fascist.
Related idea
Godwin’s law describes a rhetorical endpoint of long arguments; the reductio ad Hitlerum is the specific fallacy it points at — dismissing an idea merely because Hitler supposedly held it.