The 48-year-old entrepreneur had lashed out at Unsworth in a series of tweets after the latter dismissed Musk's proposal to build a mini-submarine to rescue the youths as a "PR stunt."
Unsworth said the custom-built device would never have fit in the cave and suggested that Musk could "stick his submarine where it hurts."
In his opening statement, Musk's attorney Alex Spiro told the jurors that his client meant no harm when he hit out at Unsworth and that his words were spoken in anger.
"The tweets are not allegations of crimes, they are joking, taunting tweets in a fight between two men," Spiro told the court.
He noted that Musk had apologized for the 'pedo guy' tweet and removed it from his feed, repeating his clients' claim that the term was a common insult in South Africa, where Musk grew up.
"Pedo guy (means) creepy old guy," in South Africa, Spiro said.
He added that Musk felt compelled to write the tweets as he was hurt by the suggestion that he was seeking publicity when he offered to help in the cave rescue.
Unsworth's attorneys challenged Musk's defense, presenting as evidence another tweet he wrote in response to a question about whether he thought the British caver was a pedophile.
"Bet ya a signed dollar it's true," he tweeted at the time. The Tesla and SpaceX CEO also said in court filings that following his spat with Unsworth, one of his aides had hired a private investigator to dig up dirt on the 64-year-old caver.
He also suggested in emails to the news website BuzzFeed that Unsworth was a "child rapist."
Unsworth's attorney Lin Wood has dismissed Musk's unsubstantiated claims as "disgusting" and an attempt to smear his client's reputation. He said he looks forward to questioning Musk under oath and to "see how a jury reacts to (his) defense strategy."
Musk, 48, was not in court on Tuesday morning when the trial got underway but Unsworth, who lives in Britain and Thailand, was present.
Unsworth is seeking unspecified damages for pain, suffering and emotional distress.
Musk had sought to have the case dismissed but a judge in October rejected the request.
The judge also rejected a bid to classify Unsworth a public figure, which would have made defamation harder to prove.
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