"Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
"That is not only not right, it is not even wrong!"
-Credited to physicist Wolfgang Pauli
'Not even wrong' is often applied, in various paraphrasings, to any theory or argument that confuses its terms to such a point that a systematic rebuttal is not feasible. While the original usage is borderline apocryphal, it can be assumed that if Pauli used it, he was using it more in the sense of 'not falsifiable', however the underlying sense of 'high-level nonsensical' remains.
This is a rather rude way of describing a person's position, and generally best avoided when speaking to them in person, even when it is true. It is also best used carefully; most nutty theories you come across are actually wrong in quite definite ways, and there is no advantage to be gained in claiming that you simply can't understand basic mistakes in science, politics, economics, logic, or any other field worth discussing.