argumentum ad fidem UK /ɑːɡjuːˈmɛntəm æd ˈfɪdɛm/US /ɑɹ.ɡjuˈmɛn.təm æd ˈfɪ.dɛm/ noun1 •(rhetoric) A fallacious argument that asserts the truth of a proposition by an appeal to pious testimony or religious revelation. formsargumenta ad fidem