lacking intelligence or good judgment; stupid; dull-witted.

lacking the power of speech (often offensive when applied to humans)

a dumb animal.

temporarily unable to speak

dumb with astonishment.

refraining from any or much speech; silent.

made, done, etc., without speech.

lacking some usual property, characteristic, etc.

performed in pantomime; mimed.

Computers . pertaining to the inability to do processing locally

A dumb terminal can input, output, and display data, but cannot process it. Compare intelligent def. 4

Nautical .

(of a barge) without means of propulsion.

(of any craft) without means of propulsion, steering, or signaling. ?

dumb down, Informal . to make or become less intellectual, simpler, or less sophisticated

to dumb down a textbook; American movies have dumbed down.

adjective

speaking , bright , intelligent , sharp , smart

adjective

at a loss for words , inarticulate , incoherent , mousy * , mum , mute , quiet , silent , soundless , speechless , tongue-tied , uncommunicative , voiceless , wordless , dense , dim-witted , doltish , dull , feebleminded , foolish , moronic , simple-minded , thick * , aphonic , blockheaded , hebetudinous , obtuse , thickheaded , thick-witted , asinine , idiotic , ignorant , inane , pantomimic , senseless , stupid , taciturn , unintelligent , unintentional