SEOUL — It wasn’t the message that came as such a surprise: It was the way Kim Yo-jong said it.

North Korea for sure was going to reject what South Korea’s president, Yoon Suk-yeol, called his “audacious initiative,” offering aid to the impoverished North in return for gradually getting rid of its nuclear program. 

The rhetoric employed by the sister of North Korea’s leader, Kim Jong-un, was shocking even by the standards of North Korean diatribes. Ms. Kim opened her rejection message by saying “it would have been more favorable for his image to shut his mouth rather than talking nonsense.”

That’s according to the English-language report by Pyongyang’s Korean Central News Agency. She has made other strongly worded statements over the years, presumably saying what her strongman brother wanted her to say. Her statement Friday pillorying Mr. Yoon, though, was wickedly vicious compared with her previous denunciations of South Korea. One earlier statement threatened revenge for supposed balloon launches that in addition to wafting propaganda she claimed had spread Covid-19 in the North.

“I am talking about Yoon Suk Yeol’s ‘commemorative speech marking August 15,’” she said, citing Mr. Yoon’s remarks on Monday, a holiday celebrating Japan’s surrender at the end of World War II. She then derided him for polls showing low levels of popularity — an extraordinary reference considering that her brother has never had to face an election while ruling with an iron fist.

The reference was so unusual as to raise the question of whether her remarks, in the original Korean language, were broadcast inside North Korea, where elections and popularity polls are otherwise unknown.

“In his situation where he is losing the public support,” KCNA quoted her as saying, “it would have been better if he had never presented himself on that occasion.”

Ms. Kim’s verbiage got progressively more colorful — and insulting. She was “sorry to say this,” as reported by KCNA, but “dogs will always bark, as a pup or an adult, and the same goes for the one with the title of ‘president.’”

Such nastiness is a matter of wonder to analysts who’ve been studying North Korean rhetoric for years. 

“North Korea’s rejection of the initiative was predictable, but I found Madame Kim’s remarks blunt and insulting to an unexpected degree,” a long-time senior diplomat in the American embassy in Seoul and also at the state department in Washington, Evans Revere, said.

“My sense,” Mr. Revere told me, “is that Pyongyang has no intention of working with the new government in Seoul, no matter what it offers.” In his view,  “the nasty tone of Kim Yo-Jong;s statement and ad hominem attack on President Yoon seem designed to tell Seoul that South-North relations will remain frozen during President Yoon’s term.”

In that spirit, Miss Kim said “the most repulsive point” about Mr. Yoon’s idea “was when he recited absurd words impertinently proposing a ‘bold and broad-based plan’ to radically improve the economy and public welfare if we would stop nuclear development and turn toward substantial denuclearization.”

Mr. Yoon’s “‘bold plan,’” she went on, mimicking his words, “is the height of absurdity as it is an impracticable one to create mulberry fields in the dark blue ocean.” It was “not a new one but a replica,” she said, of a proposal by South Korea’s president between 2008 and 2013,  Lee Myong-bak, an idea that was summarily rejected.

“The fact that he copied the policy towards the North, thrown into the dustbin of history, and called it a ‘bold plan’ shows that he is really foolish,” Ms. Kim said, professing to have been “stunned by his ‘bravery’ and excessive ignorance.”

She didn’t dwell on military exercises that American and South Korean forces are conducting for the rest of the month, but they may have played into the ferocity of her remarks.

“A knave who talks about ‘bold plan’ today and stages anti-north war exercises tomorrow,” she said, “is none other than ‘mastermind’ Yoon Suk Yeol.”

South Korean officials responded with dismay — but promised to keep looking for dialog with the North. “It is very regrettable,” the unification minister, Kwon Young-se, who is responsible for relations with the North, said. He added that Ms. Kim “distorted the audacious initiative and criticized it with a rude and undignified expression.”

The American secretary of state, Antony Blinken, and South Korea’s foreign minister, Park Jin, talking on the phone about the North Korean rebuff, formally expressed “regret” — an understatement for what had to have been their anger over a statement that was more than merely upsetting.

Mr. Yoon’s “audacious initiative,” Mr. Revere said, was “the right thing to do.” The message was “the new government in Seoul is not reflexively anti-reconciliation and is prepared to be flexible,” he said. Nonetheless, he added, North Korea “is so wedded to its missile and nuke programs, there was no other possible response from the regime.”