dense, dim, dull, dumb, obtuse, slow -- (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")
speechless, dumb -- (temporarily incapable of speaking; "struck dumb"; "speechless with shock")
dumb -- (lacking the power of human speech; "dumb animals")
dumb, mute, silent -- (unable to speak because of hereditary deafness)
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