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 (comparative dumber, superlative dumbest) (dated) Unable to speak; lacking power of speech (kept in deaf, dumb, and blind). His younger brother was born dumb, and communicated with sign language.â. Hooker to unloose the very tongues even of dumb creatures.