The story
Latin: argūmentum (“argument”, “proof”) + ad (“to”, “toward”) + populum (accusative singular of populus, “people”, “nation”) ≈ “appeal to the people”
Etymology from Wiktionary, CC BY-SA 4.0.
argumentum ad populum
Today it means: A fallacious argument that concludes a proposition to be true because many or all people believe it; it alleges that “if many believe so, it is so”. Full definition →