dumb

adjectives

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"
is derived from
temporarily incapable of speaking
"struck dumb"; "speechless with shock"
lacking the power of human speech
"dumb animals"
is similar to
without or deprived of the use of speech or words
"inarticulate beasts"; "remained stupidly inarticulate and saying something noncommittal"; "inarticulate with rage"; "an inarticulate cry"
unable to speak because of hereditary deafness