Merriam-Webster Collegiate Dictionary - dumb

 I. adjective  Etymology: Middle English, from Old English; akin to Old High German tumb mute  Date: before 12th century  1.  a. lacking the human power of speech ~ animals  b. of a person, often offensive lacking the ability to speak  2. temporarily unable to speak (as from shock or astonishment) struck ~ with fear  3. not expressed in uttered words ~ grief  4. silent; also taciturn  5. lacking some usual attribute or accompaniment; especially having no means of self-propulsion a ~ barge  6.  a. lacking intelligence ; stupid  b. showing a lack of intelligence asking ~ questions  c. requiring no intelligence ~ luck  7. not having the capability to process data a ~ terminal — compare intelligent 3a  Synonyms: see stupid  • ~ly adverb  • ~ness noun Usage:  There is evidence that, when applied to persons who cannot speak, ~ has come to be considered offensive.  II. transitive verb  Date: 1607 to make silent ; deaden would lie around, ~ed by the drugs — Norman Mailer