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1. Defying expression or description
   "unspeakable happiness";
   - indefinable, indescribable, ineffable, untellable, unutterable, undescribable
    
2. Exceptionally bad or displeasing
   "an unspeakable odour came sweeping into the room";
   - atrocious, abominable, awful, dreadful, painful, terrible, horrible
    
3. Too sacred to be uttered
   "the unspeakable name of the Deity";
   - ineffable, unnameable, unutterable, unsayable
    
4. Not capable of being said
   - unsayable

See also: bad, inexpressible, sacred, unexpressible

Encyclopedia: Unspeakable