WordWeb Online

1. Defying expression or description
   "unspeakable happiness"
   - indefinable, indescribable, ineffable, untellable, unutterable
    
2. Exceptionally bad or displeasing
   "an unspeakable odour came sweeping into the room"
   - atrocious, abominable, awful, dreadful, painful, terrible
    
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, unspeakably

Encyclopedia: Unspeakable