Merriam-Webster's Collegiate® Dictionary, 11th Edition
DUMB
I. adjective
Etymology: Middle English, from Old English; akin to Old High German tumb mute Date: before 12th century
1. lacking the human power of speech , lacking the ability to speak, temporarily unable to speak (as from shock or astonishment) , not expressed in uttered words , silent , lacking some usual attribute or accompaniment,
6. lacking intelligence ; stupid , showing a lack of intelligence , requiring no intelligence , not having the capability to process data , see: stupid dumbly adverb dumbness noun
Usage: There is evidence that, when applied to persons who cannot speak, dumb has come to be considered offensive.
II. transitive verb
Date: 1607
to make silent ; deaden