The story
Latin: argūmentum (“argument”, “proof”) + ad (“to”, “toward”) + fidem (accusative singular of fidēs, “faith”, “belief”) ≈ “appeal to faith”
Etymology from Wiktionary, CC BY-SA 4.0.
argumentum ad fidem
Today it means: A fallacious argument that asserts the truth of a proposition by an appeal to pious testimony or religious revelation. Full definition →