An argument that compares someone or something to Adolf Hitler and/or Nazism, usually as an attempt to nullify the debate or debater. The phrase is modeled on the names of classic logical arguments (e.g. reductio ad absurdum) and is pseudo-Latin for "reduction to Hitler." A: "And anyone who supports that guy is a Nazi!" B: "Geez, it's a little early for reductio ad Hitlerum, don't you think?"