US Supreme Court justices tiptoed around the offensive F-word at the center of a free speech case on Monday as they considered a challenge to a federal law that restricts trademarks on "immoral" and "scandalous" words and symbols.

The nine justices heard about an hour of arguments in a case involving Los Angeles-based clothing designer Erik Brunetti's streetwear brand "FUCT," which sounds like a profanity but is spelled differently.

The F-word word in question, which government lawyer Malcolm Stewart described as "the paradigmatic profane word in our culture," was not uttered openly in the famously decorous courtroom. Several justices signaled reservations about striking down the provision in US trademark law, which has been on the books for more than a century.

They appeared particularly concerned about limiting the government's ability to withhold trademarks featuring the most offensive words, including racial slurs.

But other justices indicated that the law is written so broadly that it violates free speech protections under the US Constitution's First Amendment. The court could well follow the course it took in 2017 when it struck down a similar law forbidding the registration of "disparaging" trademarks in case involving an Asian-American dance rock band called The Slants, a name trademark officials deemed offensive to Asians.

Justice Elena Kagan said the argument by President Donald Trump's administration in defense of the law appeared to be based largely on a commitment that the government would ban only trademarks featuring the most offensive words.

"That's a strange thing for us to do, isn't it?" Kagan asked.

Justice Neil Gorsuch followed a similar line of questioning, wondering whether the US Patent and Trademark Office's decisions on which trademarks to grant that feature offensive words are based on "the flip of a coin."

"I don't want to go through the examples. I really don't want to do that," Gorsuch added, steering clear of any profanities.

Justice Samuel Alito dismissed the suggestion by Brunetti's lawyer, John Sommer, that "FUCT" could be treated differently because it is not spelled the same as the word it sounds like.

"Oh come on, be serious. We know what he is trying to say," Alito said, though he avoided saying the brand name.

A Washington-based federal appeals court ruled in Brunetti's favor in 2017. The Trump administration appealed that ruling to the conservative-majority Supreme Court, arguing that striking down that provision would unleash a torrent of extreme words and sexually graphic images on the marketplace.

Brunetti challenged the law on free speech grounds. Having a federally registered trademark would make it easier for him to protect his brand against counterfeiters.

When the FUCT trademark application, filed in 2011, was denied, the trademark office said the brand would be perceived as the phonetic equivalent of the profanity, observing that Brunetti's products contained sexual imagery, misogyny and violence.

Brunetti said the brand's name is clever because people think it is pronounced as a profanity. The acronym, he added,also means "Friends U Can't Trust."

When the US Court of Appeals for the Federal Circuit, which specializes in intellectual property, ruled in Brunetti's favor in 2017, it acknowledged that it was "not eager" to see vulgar trademarks proliferate. But it criticized the government's attempts to police them and said the law has been applied unevenly.

Speech may not be banned even if it expresses ideas that offend, the court ruled in that case.

A ruling is due by the end of June.