"Dotard" Trump? Who insulted Donald Trump?

Kim Jong-un, The North Korean leader used the word "Dotard" for Donald Trump. He not only used this word once, but twice for Donald.

Now, the question arises, what is the meaning of "Dotard"?
Oxford dictionary describes its meaning as a person who is weak or getting weak.

It is not the first time, when North Korea used this word for Donald, but it is the other thing that this time, it gained much more attention.