Something that cannot be defined as true, yet is not necessarily false; often has a sense of truthiness to it when used in the context of a paradox.
Person 1: The first person to talk to you today is a liar.
Person 2: That would be you...
Person 1: Exactly.
Person 1: So is that true or false?
Person 2: That's an untruth, because you're projecting properties that can't exist; it's neither true nor false and you're an idiot.
Person 1: Untruth isn't a word.
Person 2: Spellcheck disagrees.