A celebrity doctor tweeted "disgusting" false rumours about First Minister Arlene Foster having an extra-marital affair to highlight any possible hypocrisy, he told the High Court.
Christian Jessen claimed his motivation for the unfounded posting was the Democratic Unionist Party's stance on marriage equality and abortion.
But counsel for the DUP leader accused him of trashing a happily married public figure's reputation with untrue social media allegations of adultery and then attempting a cover-up when she commenced libel proceedings.
The Harley Street medic, best known for presenting Channel 4 shows Embarrassing Bodies and Supersize v Superskinny, is being sued over a tweet on 23 December, 2019.
The First Minister has already told how she was left humiliated by the unsubstantiated rumour which came at a time when she was involved in talks to restore power-sharing in Northern Ireland.
The distress caused to her family was compounded by the posting being made two days before Christmas.
Mrs Foster secured judgment in default of any defence to her lawsuit.
However, a ruling on the scale of damages to award her was put on hold after Dr Jessen was ordered to travel to Belfast to be cross-examined on last-minute claims he was unaware the case had reached hearing.
Testifying in court on Friday, the defendant claimed he shut out all news for the past 12 months and moved back to his parents home because of the pandemic and his deteriorating mental health.
Defamation proceedings centre on the contents of a tweet on his account with more than 300,000 followers.
Questioned by David Ringland QC, for Mrs Foster, he told the court he believed the rumours were true, based on "hundreds" of other tweets and online websites.
"It seemed too prolifically talked about to have just been conjured from thin air, in my view," he said.
But Mr Ringland pressed him about his decision to publish false allegations to his 300,000-plus followers.
"Is it your position that you were quite entitled to tweet this disgusting tweet about the First Minister of Northern Ireland on the basis of unsubstantiated rumour that you read about online?" counsel asked.
Dr Jessen responded that his motivation was any potential hypocrisy.
"Given the extent of the rumours, and given the DUP's stance on things like equal marriage and abortion, I felt strongly that if there was a possibility these were true this is a public figure who is answerable to her public," he said.
"If there was any truth in these that sort of hypocrisy needs to be pointed out and accounted for."
It was enough for him that the rumours were possibly true, the court was told.
Mr Ringland then put to him: "You understand how shatteringly damaging it is for a happily married woman to be accused of adultery?"
The doctor acknowledged: "I'm sure it was very unpleasant for her, and I would not wish to cause her any distress."
Mr Ringland then asked: "Can we take it most of your tweets don't involve trashing a senior public figure's reputation.
"Your tweet, which is grossly offensive to the First Minister, grossly insulting, grossly defamatory, can we take it wasn't your normal fare for sending out tweets?"
His original tweet was eventually taken down on 7 January, 2020 - two weeks after it was posted.
At one point Mr Justice McAlinden challenged him about inconsistent statements about when he received the letter of claim from Mr Tweed.
Despite describing assertions he made to the solicitor as a "mistake", the judge responded: "No, it wasn't a mistake, it was a falsehood."
Maintaining there had been no intention to deceive, Dr Jessen added: "I didn't wilfully aim to mislead."
But Mr Ringland contended that he had dishonestly tried to hide a period of at least five days when the tweet remained online.
"You have lied there to try and cover up the fact that you had delayed, decided not to take down the tweet until you got advice from your lawyers," counsel claimed.
Dr Jessen insisted: "There certainly was no intention to cover up.
"As soon as I was able, when I got the letter from Mr Tweed I sought advice from my solicitors and I responded according to their advice."
He also stated how he believed all court proceedings had been stalled due to the pandemic and lockdowns.
Claiming not to have followed news events over the past year, the medic explained: "I have watched Netflix, but not current affairs."
He cited medical reasons which forced him to take time off work for the decision to shut out news coverage.
"One was quite serious mental health problems, the news hasn't been at all encouraging or cheerful over the last year."
As Dr Jessen was cross-examined about claims legal documents sent to his London apartment were never received, he described how periods of illness felt like being underwater.
Asked about podcasts he produced during that period, he said: "These were all about anxiety, depression, mental illness during lockdown."
But in closing exchanges Mr Ringland put to him: "Your answers to most of the questions on service (of documents) appear to bear no relationship to any reality that anyone in this court has ever encountered.
"Namely: the postal service doesn't work so far as you are concerned; the concierge service that you repose extreme confidence in doesn't work properly; for reasons there's been no good explanation about an active email address is something you haven't been accessing; and so on, and so on, and so on.
"Your evidence has been prevalent with lies, you have been caught out overtly on a number of occasions - not least in relation to the lie you told on January 7 with a view to try and disguise at that stage the delay in taking down the tweet."
Adjourning the case, Mr Justice McAlinden said he would decide if any further hearing on the issues raised was necessary.