The addition of -ity to an adjective results in a shift of stress to the antepenultimate syllable; that is, words in -ity are stressed on the last syllable before the -ity, even in cases where this syllable is part of another suffix (as in words in -ability and -icity).
adjective, dumber, dumbest. 1. lacking intelligence or good judgment; stupid; dull-witted. 2. lacking the power of speech (offensive when applied to humans): a dumb animal.
dumb down, to reduce the intellectual or developmental level of: to dumb down a textbook. [before 1000; Old English; c. Old High German tump, Old Norse dumbr] dumbâ²ly, adv.