Back in January, Donald Trump’s youngest son Barron went viral for extending handshakes to former US president Joe Biden and rival candidate Kamala Harris at his father’s inauguration.

Footage at the time and photographs taken from various angles, showed Barron shaking hands with Biden, who says something to the young Trump with a smile.

Many reports on social media alleged the youngest Trump child may have dropped an expletive-laden farewell to the Democratic leader and various lip readers opined differing versions what they believe was said.

On the weekend, his brother Eric finally revealed what happened in January.

“Do you remember that whole controversy where [Barron] went up to Biden … and [the media] had all these lip-reading experts [analyse it], and they’re like, ‘Barron just told Biden to go F himself’’?” Eric Trump recalled on right-wing The Megyn Kelly Show podcast.

“So one night … I call Barron, I go, ‘buddy, look, what did you actually say?’. It was something so polite I almost wouldn’t even get it right – like, ‘congratulations and best of luck to you’ – or something like that,” Eric said.

However, Eric Trump insinuated that his brother may have had a less than polite opinion of Biden.

“He’s a nice guy,” he said of Barron. 

“He’s probably thinking it. He definitely has it up here [motioning to his head], but he’s too courteous to actually go out there and say it.”

The moment between Barron and Biden happened just after Trump senior was sworn in as the 47th president of the US, but before his inaugural address.

His speech lasted just over half an hour, much of it spent rebuking Biden’s administration, while the former president sat just metres away.

Unlike his brothers Eric and Donald jr, Barron Trump has maintained a low profile throughout his father’s presidencies.

The 19-year-old is studying business at New York University and recently transferred from the university’s main campus in Manhattan to its smaller Washington DC campus.

It is thought he has moved into the White House.

While Barron – unlike his father and brothers – has no social media presence, Donald Trump has previously praised his social media savvy and influence on his presidential campaign.

Donald Trump is father to five children to three women. He shares sons Eric, Donald Jr and daughter Ivanka with his late ex-wife Ivana Trump. He also has a younger daughter, Tiffany, with ex-wife Marla Maples and Barron with wife Melania Trump.