according to that quote, people should stay quiet when people think they are morons, why? being thought a moron makes people respect you and your opinion less, ignore any testimony of yours and basically ignore anything you have to say.
the first impression people will ever have of you is the language you speak. if you are boistrous (sp?) and can pronounciate very articulately what you are saying, they will think very highly of you and your opinions. swearing makes people think you are not the brightest bulb in the box