dumb
adj.
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|dumb officials make some really dumb decisions|he was either normally stupid or being deliberately obtuse|worked with the slow students
Synonym: dense, dim, dull, obtuse, slow
adj.
temporarily incapable of speaking
struck dumb|speechless with shock
Synonym: speechless
adj.
lacking the power of human speech
dumb animals
adj.
unable to speak because of hereditary deafness
Synonym: mute, silent