Lots of dumb words this week . . . such as ditsy, dolt, dopey, dork, dotard, duffer, dullard, dumbhead, dumbo, dummkopf, and dunce.
Of course, one should be careful with dork, as "a stupid or foolish person" is its second meaning; the first is "the penis" (OED verified). I first learned this from Gary Larson, in the preface to one of his Far Side collections. So choose your insults wisely.