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Synonyms for Dumb (same or very similar meaning)

WordNet sense 1 (slow to learn or understand; lacking intellectual acuity):
dense, dim, dull, obtuse, slow

WordNet sense 2 (lacking or marked by lack of intellectual acuity):
stupid

WordNet sense 3 (unable to speak temporarily):
speechless

WordNet sense 4 (unable to speak because of hereditary deafness):
mute, silent

WordNet sense 5 (without or deprived of the use of speech or words):
unarticulate


From the ODE community, based on WordNetadd/edit