The inspiration for cleaning up his vocabulary came from God himself, Duterte said, as he was flying back to the Philippines from a state visit to Japan.
"Everybody was asleep, snoring, but a voice said... 'If you don't stop (cursing), I will bring this plane down now.' And I said, 'Who is this?' So, of course, it's God," the President recounted after he landed Thursday at Davao International Airport.
"So I promised God not to express slang, cuss words and everything," said Duterte -- who recently questioned the existence of God -- adding that a "promise to God is a promise to the Filipino people."
When those in attendance for his arrival speech began to applaud his declaration, he stopped them.
"Don't applaud too much, I might fail," he said.
Duterte's vernacular has grabbed global headlines for its lack of, well, decorum.
He infamously called both President Barack Obama and Pope Francis a "son of a bitch," declared that the U.S. ambassador to the Philippines was a "gay son of a bitch" -- adding that the ambassador had "pissed him off" -- and told the European Union to "f*ck off," accompanied by the universally understood hand gesture.
Now the question is... how long can he keep his promise, dammit?