The other side carries the burden here, and they can't discharge it. A claim is only provable if it has defined truth conditions. "Poopsalotofpee" specifies none, so no evidence could ever confirm or refute it. That's not a technicality, it's the whole game: affirmatives must establish a proposition, not an utterance. And notice the asymmetry. If I merely withhold belief, I need no proof. If my opponent asserts, they need proof, and they cannot point to a single test that would distinguish a world where they're right from one where they're wrong. So the negative wins by default.