I know what a "dotard" is.  It's not an obsolete or archaic word.  That any educated person wouldn't know speaks badly of their exposure to literature and the English language. 

Still, usage like this reminds me of the kind of messaging used during the Cold War, where quaint or archaic expressions were often used in insults.  It kind of takes the edge off an insult when a lot of people don't understand what it means.