backward, birdbrained, defective,
dim,
dim-witted, dopey, dull, dumbbell, dumbo, dumdum,
dummy, exceptional, feeble-minded, gorked, half-witted, held back, imbecile, lamebrained, mentally defective, moronic, numbskull, opaque, pinhead, retardo, sappy, simple, simple-minded, slow, slow-witted, stupid, subnormal, touched, underachieving, weak, yo-yo, impaired, inarticulate, incoherent, mousy,
mum, mute, quiet, silent, soundless, speechless, tongue-tied, uncommunicative, wordless.