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.
In Britain 'dumb' is seldom used to mean stupid. It was mostly only used to mean mute e.g 'deaf and dumb'. However 'dumb' in the mute sense is probably politically incorrect nowadays, a bit like calling a short person a 'dwarf'.