1-sisimumuI couldn't speak; I was dumb with shock — Andizange ndibe nakho ukuthetha; ndasuka ndasisimumu kukothuka
2ngokungekho sikweni-sisiyathaStop being dumb and think! — Yeka ukuba sisiyatha oku ucinge!→ Synonym of stupid
In the past 'dumb' was used to mean "unable to speak". It is better not to use it in this sense because it can hurt people's feelings. Instead use a phrase such as 'speech-impaired'.