WordWeb Online

1. Too sacred to be uttered
   - ineffable, unnameable, unspeakable, unsayable
    
2. Defying expression or description
   "unutterable contempt";
   - indefinable, indescribable, ineffable, unspeakable, untellable, undescribable
    
3. Very difficult to pronounce correctly
   "unutterable consonant clusters";
   - unpronounceable

See also: incommunicative, inexpressible, sacred, uncommunicative, unexpressible