The Tesla CEO told jurors that he called Vernon Unsworth "pedo guy" because he was upset the diver had belittled his efforts to help by building a mini-submarine to transport the boys. Unsworth called it nothing more than a "PR stunt" and said Musk could stick the contraption "where it hurts." The sub was never used.
"It was wrong and insulting, so I insulted him back," the billionaire said in federal court in Los Angeles. "It was an unprovoked attack on what was a good-natured attempt to help the kids."
Unsworth's lawyers argue that Musk's tweet overshadowed what should have been one of the diver's proudest moments and left him no choice but to sue.
At the time, Musk said he didn't know Unsworth had helped with the rescue. He said his tweet wasn't intended as an actual allegation.
"Just as I hadn't literally meant he was a pedophile, he didn't want to shove a sub up my ass," Musk said, provoking snickers in the courtroom.
Unsworth is seeking unspecified damages for pain, suffering and emotional distress. The defense has resisted efforts to turn over documents showing Musk's wealth but says his net worth exceeds $20 billion.
Musk's lawyer, Alex Spiro, said earlier that Unsworth deserves nothing for what he called "joking, taunting tweets in a fight between men."
The shame and mortification Unsworth said he experienced is undercut by the attention he received after the rescue, Spiro said, including honors from the Thai king and British prime minister and offers from agents and film crews.
"There's a little more to this Twitter insult, and you'll hear it," Spiro told jurors.
Musk has apologized for lashing out at Unsworth and removed his tweet, but he also has defended using the phrase.
"'Pedo guy' was a common insult used in South Africa when I was growing up," Musk said in a court declaration. "It is synonymous with 'creepy old man' and is used to insult a person's appearance and demeanor."
Unsworth's lawyers laughed off that explanation, saying Musk responded to a question about whether he had accused Unsworth of being a pedophile with another tweet that said, "Bet ya a signed dollar it's true."
The diver's lawyers also said Musk hired private investigators to dig up evidence Unsworth was a child molester, which they never found.
Musk suggested in emails to BuzzFeed that Unsworth was a "child rapist" and had moved to northern Thailand to take "a child bride who was about 12 years old at the time." He provided no evidence.
The lawsuit is not the first time Musk's tweets have landed him in hot water.
Musk and Tesla reached a $40 million settlement with the Securities and Exchange Commission last year over allegations he misled investors with a tweet declaring he had secured financing to buy out the electric car maker. He agreed in the settlement to have future tweets about the company screened.
He went back to court on accusations he violated that agreement by tweeting a misleading figure about how many cars Tesla would manufacture this year. The SEC sought to hold him in contempt of court, which led to a new agreement imposing tighter controls on Musk's tweets about the company.
The cave drama played out for more than two weeks in summer 2018.
Musk and engineers from his SpaceX rocket company custom-built a mini-submarine, which was heavily publicized but never used.
Unsworth, a diver and caving expert whose advice was considered crucial in the rescue operation, said the sub would never have fit in the cave.