The Philippine president, who once called the pope a "son of a bitch" and told Barack Obama to "go to hell", says he has promised to God he will not use expletives again.
President Rodrigo Duterte's profanities have become his trademark, especially when threatening to kill drug dealers as part of his war on illegal drugs that has left thousands dead since he took office at the end of June.
Mr Duterte made the pledge on arrival in his southern home town of Davao city from a trip to Japan.
While flying home, he said he was looking at the sky while everyone was asleep, some snoring, and he heard a voice that said "'if you don't stop epithets, I will bring this plane down now".
"And I said, 'Who is this?' So, of course, 'it's God'," he said.
"So, I promise God to ... not express slang, swear words and everything. So you guys hear me right always because (a) promise to God is a promise to the Filipino people."
Mr Duterte's vow was met with applause, but he cautioned: "Don't clap too much or else this may get derailed."
He shocked Roman Catholics last year when he fired off an expletive while expressing his disgust over a monstrous traffic jam that trapped him while Pope Francis was visiting Manila.
"I wanted to call. 'Pope, you son of a bitch, go home.' Don't visit here anymore," he told a mob of supporters, some of whom laughed.
He later apologised after Filipino bishops expressed shock and outrage.
It is not certain if the 71-year-old president, who has been compared to US Republican presidential candidate Donald Trump because of his brash language, can keep his promise.
Mr Duterte made a similar pledge in June when it became evident that he had won the May 9 presidential elections overwhelmingly on a pledge to end crimes, especially illegal drugs, and corruption.
He said then that he was enjoying his last moments as a "rude person" because "when I become president, when I take my oath of office ... that will be a different story. There will be a metamorphosis".
It did not take long for Mr Duterte to break the promise.
He has repeatedly levelled tirades against Mr Obama, UN secretary-general Ban Ki-moon, a few outspoken opposition politicians and human rights advocates, while heaping praise on Chinese and Russian leaders.
There were no expletives in his speech late on Thursday, but Mr Duterte still sounded mean towards critics.
When asked for his message for a Filipino beauty queen who won the recent Miss International pageant, Mr Duterte said he was proud.
"Many Filipinas are beautiful, but all of you there in the human rights commission are ugly," he added.
Asked if the days of his swearing at the US and the EU are over, Mr Duterte did not answer clearly.
"I do not want anybody reading my mind because I couldn't make a smart move anymore. But it's all calibrated, it's all about timing," Mr Duterte said.