Unable to be discovered, undiscoverable, not discoverable.
Incapable of being articulated or expressed; indescribable, inexpressible.
Not utterable; incapable of being physically spoken or voiced; unpronounceable.
Not allowed to be spoken; taboo, unspeakable.
Extremely bad or objectionable; unspeakable.
Something which is unutterable (incapable of being physically spoken, incapable of being articulated or expressed, etc.).