An academic in South Africa has cast doubt on Elon Musk's defence that the term "pedo guy" is a common insult in Pretoria and therefore not defamatory, ahead of Tuesday's hearing and his legal counsel doubling-down on the stance to the ABC.

Key points: Elon Musk is trying to have a defamation case against him thrown out before trial

Elon Musk is trying to have a defamation case against him thrown out before trial An academic rejected Mr Musk's statement that the 'pedo guy' insult he levelled against a British diver is a common South African insult

An academic rejected Mr Musk's statement that the 'pedo guy' insult he levelled against a British diver is a common South African insult Mr Musk spent more than $74,000 on a private investigator to "dig deep" into the diver's past

A Los Angeles court will decide on Tuesday (7:30am AEDT) whether to go ahead with a defamation case against Mr Musk, brought by British caver Vernon Unsworth, who played a key role in last year's Thai cave rescue operation.

In a television interview, Mr Unsworth suggested Mr Musk's offer to provide a submersible rescue pod was grandstanding and said Mr Musk "can stick his submarine where it hurts".

The founder of Tesla and SpaceX took to Twitter in response, ending a volley of angry tweets with "sorry pedo guy, you really did ask for it" — he later apologised and deleted the tweets.

Mr Unsworth sued two months later in a Los Angeles federal court, saying Mr Musk falsely branded him a paedophile and child rapist.

Mr Musk has not provided any evidence to back his allegation and is instead arguing that he relied on information provided by a private investigator which he did not know to be false at the time and did not intend to publicly brand Mr Unsworth a paedophile.

Mr Unsworth has strongly denied any accusation of paedophilia and has sued for defamation, looking for $111,000 in damages.

British cave diver Vernon Unsworth is suing Mr Musk for defamation. ( AP: Sakchai Lalit )

However, Mr Musk is trying to have the case thrown out before trial on several legal grounds.

"Mr Musk testified that 'pedo guy' was a common insult used in South Africa during his youth," wrote Mr Musk's attorneys, in court documents.

"It is synonymous with 'creepy old man' and aimed at mocking a person's appearance and demeanour, not an accusation of paedophilia," the attorneys wrote.

The ABC contacted several linguists from universities in Pretoria — where Mr Musk lived until he was 17 years old — to check the claim.

Most did not feel appropriately qualified to address the issue, but Molly Brown, the head of the Department of English at the University of Pretoria, said the term was not well known.

"I think I can confirm that 'pedo-guy' is not a commonly used insult in South Africa," she told the ABC.

"A quick look at the local twitter sphere confirms this … the only tweets I can find in support of Mr Musk's assertion come from men who attended his school," she said.

"This might mean that the term was current in his particular school environment."

"I also think that just as most of us would rather die than dress as we did at school, so the average person abandons school slang as a means of public communication within a surprisingly short time after leaving school behind."

Musk spends tens of thousands on private investigator

Elon Musk visited the Thai cave where the 12 boys of the Wild Boar soccer team and their coach were trapped, to drop off a mini submarine his team made. ( Twitter: Elon Musk )

In a statement to the ABC, Alex Spiro, one of the lawyers representing Mr Musk, doubled down on the supportive tweets as evidence that "the term or insult 'pedo guy' was commonly used in Mr Musk's school in South Africa".

"Mr Musk deleted the tweet and apologised," he said.

While part of the legal appeal seems to try to distance Mr Musk from the paedophile allegation.

Mr Musk spent more than $US50,000 ($74,000) on a private investigator, who offered his services to "dig deep" into Mr Unsworth's past.

The investigator travelled to Thailand and sent back reports with unverified claims, which Mr Musk appears to have taken as fact.

"The investigator reported that Mr Unsworth was a fixture in Pattaya Beach, Thailand — a locale notorious for prostitution and child trafficking, that he had a taste for young Thai girls," wrote Mr Musk's attorneys in the court documents.

The attorneys wrote that according to the investigator: "He whore-mongered his way through the go-go bars of Thailand, that his only friends were his 'sexpat' peers, and that he married his Thai wife when she was a teenager, after starting a relationship when she was a young girl."

Again, no evidence was presented to Mr Musk, or to the court in the recent documents.

Into the cave Cave diving is perilous at the best of times but finding and freeing the 12 Thai boys and their football coach from Tham Luang posed extreme challenges. Read more Read more

In their response to the court, Mr Unsworth's lawyers say that, contrary to the investigator's reports, he met his wife in London in 2011 when she was 32 years old and he only made his first trip to Thailand to visit her in 2018.

"Like the bully that he is, Musk chose to lash out publicly at the criticism only by falsely attacking Unsworth, a relatively unknown individual, and publicly challenging him to sue for libel," Mr Unsworth's attorney Lin Wood wrote.

"Musk's motion is based principally on the antithetical bases that, on the one hand, he was not calling Unsworth a paedophile, while on the other hand, he did not harbour serious doubts as to whether Unsworth was actually a paedophile."

Based on these verbal reports from the so-called investigator, Mr Musk emailed comments to a BuzzFeed News reporter that were meant to be off the record, referring to Mr Unsworth as a "child rapist".

Mr Unsworth suggested Mr Musk's offer was grandstanding, adding he could "stick his submarine where it hurts". ( Twitter: Elon Musk )

Mr Musk's lawyers are now arguing the billionaire should not be held liable for his comments because he did not believe them to be false.

"Because Mr Musk based this tweet on information concerning which he did not have serious doubts, Mr Unsworth cannot establish it was written with actual malice," wrote Mr Musk's attorneys.

"Although it turns out that the investigator lacked solid evidence of Mr Unsworth's behaviour, that does not matter here."

In their response to the court, Mr Unsworth's lawyers say Mr Musk did not vet the investigator, James Howard-Higgins, for reliability and say Mr Musk himself now acknowledges that Mr Howard-Higgins was a "con man … just taking us for a ride".

The defamation case against Mr Musk has been set for December 2.

