dumb (adj.)

1.unable to speak temporarily"struck dumb" "speechless with shock"

2.unable to speak because of hereditary deafness

3.lacking the power of human speech"dumb animals"

4.(colloquial)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"