uninvestigable
Uninvestigable is an adjective used to describe something that cannot be investigated or examined. This can be due to a variety of reasons, such as a lack of evidence, the passage of too much time, or the inherent nature of the subject itself. For example, a crime may become uninvestigable if all witnesses have died and there are no physical clues remaining. Similarly, certain philosophical or metaphysical questions might be considered uninvestigable by some as they lie beyond the scope of empirical observation or scientific inquiry.
The term implies a definitive inability to proceed with an investigation, suggesting that any attempt to do