Definitions
1.AdjectiveNot utterable; incapable of being physically spoken or voiced; unpronounceable.
2.AdjectiveIncapable of being articulated or expressed; indescribable, inexpressible."unutterable anguish"
3.AdjectiveNot allowed to be spoken; taboo, unspeakable.
4.AdjectiveExtremely bad or objectionable; unspeakable.
5.NounSomething which is unutterable (incapable of being physically spoken, incapable of being articulated or expressed, etc.).