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Referencing human rights criticisms of his policies, Duterte said that he wasn't afraid to go to jail, and apparently used the anecdote to illustrate that.
"One fight there, another here -- at the age of 16, I killed someone. A person, really. During a fight. Stabbing. That was when I was 16 years old, just because we just looked at each other," he added, prompting laughter from the audience.
The controversial leader, who was elected largely on a platform of zero-tolerance against the Philippines' drug scourge, suggested that his approach to the war on drugs, mirrored that of his young self.
His approach was "even more so as president," he said. "You f**k with my countrymen, I won't let you off the hook. Never mind about the human rights advocates."
Duterte's spokesman Harry Roque told CNN affiliate CNN Philippines in a text: "I think it was in jest. The President uses colorful language when with Pinoys overseas," using another word for Filipinos.
Duterte is due to meet US President Donald Trump who arrived in Vietnam Friday for the Asia-Pacific Economic Co-operation (APEC) summit. Trump travels to the Philippines on Sunday for meetings with the Association of Southeast Asian Nations (ASEAN).
Previous allusion?
"There was a tumultuous fight in the beach," he said in the interview.
"We were young men then and we went to this beach and we were drinking and suddenly there was this ... maybe I stabbed somebody to death ... something like that."
History of killing
"In Davao, I used to do it personally. Just to show the guys that, if I can do it, why can't you?" Duterte said.
"And (I'd) go around Davao with a motorcycle, with a big bike around, and I would just patrol the streets, and looking for trouble also. I was really looking for an encounter so I could kill."
He's said that he had taken part in at least one raid on suspected kidnappers during his tenure as mayor, which ended in the suspects' deaths.
More than 1,000 people were killed by the DDS while Duterte was mayor from 1988 until 2013. His spokesman, Martin Andanar, has denied that he was involved.
In a televised speech, also in late 2016, he said had previously thrown someone off a helicopter and threatened the same fate to anyone misusing public funds.
When asked if the event actually happened -- and if the person killed was a kidnapper, as local media was reporting -- Martin Andanar, the presidential communications secretary, told CNN: "It happened; he said it."
Duterte later walked back the remarks, claiming he was joking.
CNN's Samantha Beech and Janie Octia contributed to this report.