An unlikely word is trending on Twitter on Friday morning: dotard.

The insult was a riposte from Kim Jong Un after US President Donald Trump nicknamed the North Korean leader "Rocket Man" at the UN General Assembly.

The unusual word left many people scratching their heads, and Google searches for dotard went through the roof.

So, what does it actually mean? The Twitter-savvy dictionary Merriam-Webster was quick to come to people's aid.

Merriam-Webster defines dotard as "a person in his or her dotage," with dotage meaning "a state or period of senile decay marked by decline of mental poise and alertness."

It stems from the Middle English word doten, which was used to describe an imbecile at the time of its introduction in the 14th century.

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The Washington Post's Tokyo bureau chief, Anna Fifield, though, was quick to point out that the Korean definition of Kim's barb was "lunatic old man."

According to Google's Ngram Viewer, which allows users to track the popularity of words through time, use of the word dotard peaked in English corpus in the early 18th century.

Obviously, this word is slightly dated — and some people were left perplexed:

Some people thought Kim came out on top of the exchange:

Others just straight-up agreed with the North Korean leader: