indeterminable

The adjective indeterminable describes something that can't be known for sure, like what grade you would've gotten on a paper you never turned in.

To determine is to calculate, measure, define, or decide something with certainty. Something that's indeterminable cannot be determined. It's not just a matter of "we don't know yet." Instead, it usually means that the system, data, or logic itself prevents a clear answer from ever being found. Vague language in a legal contract might be so ambiguous that a court cannot determine its true intent. The truth or falseness of a statement such as "This statement is false" can never be determined: If it's true, it's false; if it's false, it's true.