Not necessarily 'dumb' but perhaps a psychiatric issue. A guy I know is a very clever tradesman who has brilliant skills in electrical design and implementation but he can't help himself in terms of mouthing off about potentially inflammatory subjects at the most inappropriate times and surroundings. His employers have learnt to limit his exposure to senior clients due to his abrasive manner as there have been comments along the lines of 'don't send that guy out to our site again'.
A common friend with psychology qual's has identified his behaviour as a recognised affliction, and definitely not symptomatic of a low IQ.
A common friend with psychology qual's has identified his behaviour as a recognised affliction, and definitely not symptomatic of a low IQ.