dumb in WordNet English dictionary

1. slow to learn or understand; lacking intellectual acuity
   "so dense he never understands anything I say to him"; "never met anyone quite so dim"; "although dull at classical learning, at mathematics he was uncommonly quick"- Thackeray; "dumb officials make some really dumb decisions"; "he was either normally stupid or being deliberately obtuse"; "worked with the slow students"
   dense  dim  dull  dumb  obtuse  slow 
   
2. unable to speak because of hereditary deafness
   dumb  mute  silent 
   
3. temporarily incapable of speaking
   "struck dumb"; "speechless with shock"
   speechless  dumb 
   
4. lacking the power of human speech
   "dumb animals"
   dumb