North Korea's riposte to Donald Trump had many native English speakers reaching for their dictionaries.
A dispatch described the US president as "the mentally deranged US dotard".
"Dotard" means a person in a feeble and childish state due to old age.
It is a translation of "neukdari", a Korean word that refers to elderly people in a derogatory manner.
The Korean Central News Agency transmitted Kim Jong Un's statement verbatim.
It followed Mr Trump's speech at the UN this week mocking Mr Kim as a "Rocket Man" on a "suicide mission", and saying if the US is "forced to defend itself or its allies, we will have no choice but to totally destroy North Korea".
North Korea's top diplomat has reportedly warned that his country may test a hydrogen bomb in the Pacific Ocean.
Foreign Minister Ri Yong Ho told reporters in New York that a response "could be the most powerful detonation of an H-bomb in the Pacific", according to South Korean media.
Yonhap news agency reports that Ri added: "We have no idea about what actions could be taken as it will be ordered by leader Kim Jong Un."
Earlier, the North Korean leader had said that Trump was "unfit to hold the prerogative of supreme command of a country" and described the president as "a rogue and a gangster fond of playing with fire".
"I will make the man holding the prerogative of the supreme command in the US pay dearly for his speech calling for totally destroying the DPRK," said the statement carried by the Korean Central News Agency in a dispatch issued from Pyongyang on Friday morning.
DPRK is the abbreviation of the communist country's official name, the Democratic People's Republic of Korea.
The statement responded to Trump's combative speech at the UN General Assembly on Tuesday where he mocked Kim as a "rocket man" on a "suicide mission" and said that if "forced to defend itself or its allies, we will have no choice but to totally destroy North Korea".
Kim Jong Un described Trump's comments as "mentally deranged behaviour".
He said the remarks "have convinced me, rather than frightening or stopping me, that the path I chose is correct and that it is the one I have to follow to the last".
He added that he is "thinking hard" about his response and that Trump "will face results beyond his expectation".
South Korea said Kim's rebuke against Trump marked the first time a North Korean leader had directly issued a statement to the international community under his name.
Seoul's Unification Ministry said neither of the two men who ruled North Korea before Kim Jong Un - his father, Kim Jong Il, and his grandfather and national founder Kim Il Sung - issued any similar statement.
Ministry spokesman Baik Tae-hyun said North Korea should stop provocations that would "lead to its own isolation and demise".
Meanwhile, a US nuclear and security analyst said his "biggest fear" was that North Korea's recent statements signal the country might conduct an atmospheric test of a nuclear weapon atop a missile.
Such tests have been conducted in the past by the United States and China, but not in recent decades.
Vipin Narang, a nuclear expert at the Massachusetts Institute of Technology, said this would be worse than testing a longer range for an intercontinental ballistic missile, which is another scenario the North may be considering.
He said such a test could pose a danger to shipping and aircraft, even if the North declared a "keep out" zone, and would pose a risk to people if something went wrong.
"We are talking about putting a live nuclear warhead on a missile that has been tested only a handful of times. It is truly terrifying if something goes wrong," he added.
In Hawaii, state lawmakers have been urging emergency management officials to update Cold War-era plans for coping with a nuclear attack.
Hawaii Emergency Management Agency administrator Vern Miyagi said that while an attack is not likely, the threat cannot be ignored.
The state has plans to reintroduce monthly tests of an attack-warning siren.