unutterable🔊

Not utterable; incapable of being physically spoken or voiced; unpronounceable.

Incapable of being articulated or expressed; indescribable, inexpressible.

Not allowed to be spoken; taboo, unspeakable.

(figuratively) Extremely bad or objectionable; unspeakable.

Something which is unutterable (incapable of being physically spoken, incapable of being articulated or expressed, etc.).