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.
Dummy is a derived term of dumb. From (etyl) dumb, from (etyl) . In ordinary spoken English, a phrase like He is dumb is interpreted as He is stupid rather than He lacks the power of speech. The latter example, however, is the original sense of the word.