It seems every one was stumped. Apparently it's was a translation of the word "senile old man" a common description across most cultures. The hiccup appears to be to be the use of an old English-Korean dictionary which the NK translator used. Tells you how backward the country is and at the same time the ability to test a hydrogen bomb. Scary when an outdated dictionary is used by a regime who are also preparing to weaponise their missiles with the hydrogen bomb.
Remind me of the numerous time Islamic Nutjobs jobs have blown themselves.
This sounds to me like western propaganda to belittle North Korea, I don't buy it at all. The "mentally deranged dotard" snipe is directly attributed to KJU - there is no way such a statement can come out without being being scrubbed by all sorts of propaganda and translation experts in North Korea.
North Korea might have severely limited its citizens from accessing the internet, but I'm pretty sure a minority of special people are more than adequate to do a 5 second google to find the word "senile" if they had needed to. Besides there has to be at least 1 person in a country of 25 million with good enough English competency to translate simple words like idiot or senile. The idea that some North Korean is busy flipping a dusty 1950s English dictionary to translate their supreme leader's statement is ludicrous.
The use of dotard is a specially devised rheotorical instrument to counter Trump's equally colorful "rocketman" narrative. Its success is apparent in the sense that the whole world is now talking about rocketman and dotard with google reporting a spike in search for these words. These two fellows are hurling serious weapons grade propaganda/marketing at each other.