1.Definition: lacking the power of human speech

"Dumb animals"

2.Definition: slow to learn or understand; lacking intellectual acuity

"Dumb officials make some really dumb decisions"

Related Adjective(s):dense, dim, dull, obtuse, slow

3.Definition: temporarily incapable of speaking

"Struck dumb"

Related Adjective(s):speechless

4.Definition: unable to speak because of hereditary deafness

Related Adjective(s):mute, silent