Reductio ad Hitlerum
Etymology
A mock-Latin phrase coined by philosopher Leo Strauss in his 1953 "Natural Right and History," modeled on the logical form "reductio ad absurdum" ("reduction to the absurd"). It substitutes Hitler for the absurd conclusion.
noun phrase · /rih-DUK-tee-oh ad HIT-ler-um/
The fallacy of dismissing an idea by claiming that Hitler, the Nazis, or some reviled figure also held it.
What it names
A specific bad argument — guilt by association with the worst possible company, in place of actual reasoning.
Example
"The Nazis also supported animal welfare" is a reductio ad Hitlerum — the source doesn’t make the position wrong.
Related idea
Godwin’s law is the related internet observation that online arguments trend toward Nazi comparisons; this term names the fallacy that so often results.