NEW YORK (NYTIMES) - It began as a chant at a Nascar race. It became an inside joke among many Republicans that spread to T-shirts and even to the floor of Congress. And now it has entangled Southwest Airlines in the nation's political tussles.

The phrase "Let's go Brandon", which is understood to be code for swearing at President Joe Biden, was uttered over the intercom by a Southwest pilot during a flight last Friday (Oct 29), a reporter for The Associated Press wrote in an article about the spread of the phrase.

The reporter, Ms Colleen Long, who was on that flight, added that it prompted "audible gasps from some passengers".

As word of the remark spread on social media, many threatened to boycott the airline. Others pledged their support to Southwest because of the pilot's remark. Southwest apologised to customers on Sunday and said it was conducting an internal investigation.

"Southwest does not condone employees sharing their personal political opinions while on the job," the company said in a statement emailed to The New York Times. The airline would not say if the pilot had been suspended for making the remark, adding that it does not comment on an employee's status.

The viral moment began in early October at a Nascar race in Alabama that was broadcast on NBC.

As a crowd appeared to be cheering on the driver Brandon Brown, an NBC reporter interviewing him suggested that people were chanting "Let's go, Brandon", but it became clear that they were actually saying a four-letter epithet and then "Joe Biden".

Since then, the phrase has exploded in popularity: Lawmakers, musicians and former President Donald Trump's campaign PAC all have used it with a joking tone.

Texas Governor Greg Abbott said the phrase on Twitter on Oct 22. Republican representative Bill Posey said it on the House floor during a speech criticising the Biden administration.

Billboard reported that artist Loza Alexander made its Hot 100 chart for the week of Oct 30 with a song titled Let's Go Brandon. But others, including many Democrats, do not find the phrase funny. Twitter users who called for a boycott of Southwest said the airline should punish the pilot.

Political viral moments have become increasingly common in the digital age. During the 2020 election, a Trump campaign news conference held in the parking lot of a landscaping company inspired bumper stickers, memes, a documentary and even a charity run.

Dr Karen North, a professor of digital media at the University of Southern California, who worked for the Clinton administration, said that a moment like the "Brandon" phrase "has the fun of being an inside joke or meme and the power of being a rallying cry at the same time".

But these moments seem to have an ever-shorter shelf life, she said.

"Because new trends and memes spread so much more quickly," she added, "people have something new to jump to more quickly."