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.
Today, most of the time someone or something is described as dumb when it shows a lack of thought, like your dumb idea of wearing flip-flops in a snowstorm. Today, dumb still means unable to speak, but it doesn't have anything to do with intelligence. Its other meaning, however, does.