He is being sued by Vern Unsworth, who aided the rescue of 12 boys from Thailand's Tham Luang caves.
The two clashed over how to free the boys in an exchange that led to Tesla's chief calling Mr Unsworth "pedo guy".
Mr Musk's lawyers said the "insult" had been made in response to Mr Unsworth's own disparaging remarks.
Mr Unsworth is from St Albans, Hertfordshire, but now lives near Chiang Rai in Thailand.
The "vituperative" exchange between Mr Musk and Mr Unsworth took place during frantic attempts to rescue the 12 boys and their coach from deep within the partially flooded caves in July 2018. Mr Unsworth helped recruit experienced UK cave divers, who were instrumental in freeing the boys.
Mr Musk mobilised a group of Tesla engineers to help with the rescue effort and left behind a specially designed mini-submarine that he claimed could help transport the children out of the caves. The submarine was never used to free the boys.
In an interview on CNN, Mr Unsworth ridiculed the submarine calling it a "PR stunt".
Responding in a series of tweets, Mr Musk gave more details about how the submarine might work and posted the "pedo" comment.
When quizzed for his response, Mr Unsworth said he was considering legal action.
Soon after, Mr Musk apologised and deleted the offending tweets saying he had acted "in anger".
He added: "His actions against me do not justify my actions against him, and for that I apologise."
But the row escalated after Mr Musk referenced his initial insult in a separate Twitter exchange.
In this, he said it was "strange" that Mr Unsworth had not sued him over the allegation despite being offered free legal advice.
Soon after, Mr Unsworth's lawyers posted a series of tweets criticising Mr Musk and his characterisation of their client.
Documents prepared by Mr Musk's attorney said he responded to this via "off the record" emails to Buzzfeed journalists seeking comment in which he issued further "insults", some of which alleged Mr Unsworth had a "child bride".
Despite being labelled as "off the record", the emails were published and led to the lodging of the defamation claim in a California court.
Mr Musk wants the court to throw out the defamation claim because, his lawyers say, his comments were "non-actionable opinion".
The legal team said the Tesla chief had never met Mr Unsworth so his comments had no "factual basis".
Instead, the words were "over-top-insults not driven by first-hand knowledge".
Mr Musk's lawyers argue at length that because the "schoolyard spat" blew up on Twitter, which they say is "infamous for invective and hyperbole", no-one could reasonably believe the comments were truthful.
The fact that they were on social media served to underline how different they were from a proper press expose or criminal complaint, the lawyers add.
Neither Mr Unsworth nor his own legal team have responded to requests for comment.
