Vernon Unsworth, who helped rescue youth soccer players trapped underwater in a Thailand cave, is taking his defamation case against Musk to jurors Tuesday in Los Angeles federal court.
"Pedo guy was a common insult used in South Africa when I was growing up".
Mr Musk later apologised for lashing out at Mr Unsworth on Twitter after the diver belittled Mr Musk's efforts to build a tiny submarine to save the trapped boys as a "PR stunt".
He must prove the tweets were false, that Musk did not use reasonable care to determine if they were true, and that people reasonably understood them to mean he was a pedophile. "It is synonymous with "creepy old man, ' and is used to insult a person's appearance and demeanour".
Lawyers for Musk have said the tweets were opinion, not statements of fact, and that Unsworth knew of no one who thought he was a pedophile based on them.
The lawyers also said he hired private investigators to dig up evidence Mr Unsworth was a child molester, which they never found, according to Mr Unsworth's lawyers.
The case against Mr Musk got a boost when Judge Wilson last month said Mr Unsworth's sudden fame from the rescue did not make him a "public figure", meaning he did not need to show that Mr Musk acted with "actual malice" when posting his tweets.
Mr Musk and Tesla reached a $40 million settlement with the Securities and Exchange Commission previous year on allegations he misled investors with a tweet declaring he had secured financing to buy out the electric vehicle maker.
Mr Unsworth played a lead role in the rescue of 12 boys and their football coach from the Tham Luang Nang Non cave in Thailand in July 2018.
The episode began after Mr Musk offered a mini-submarine from his SpaceX rocket company to help with the cave rescue. The device was heavily publicised 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's tight spaces.
Mr Musk responded two days later with his series of tweets.
On Monday, the judge refused a defense request to reverse a decision allowing Unsworth's attorneys to introduce an email Musk sent to the news website BuzzFeed in which Musk chided a reporter there to "stop defending child rapists". He provided no evidence.
Mr Unsworth is seeking unspecified damages for pain, suffering and emotional distress.
The defence has resisted efforts to turn over financial records to show Mr Musk's wealth but has stipulated his net worth exceeds $20 billion (£10.57 billion).