US first lady Melania Trump on Wednesday publicly rebuked a professor for using her 13-year-old son Barron Trump to make a point during an impeachment hearing against president Donald Trump.

Constitutional law professor Pamela Karlan used the name of Barron Trump, the son of Donald and Melania Trump, to make a pun regarding how the Constitution imposes distinctions between a monarch's power and that of a president. The scholar argued that while Trump views himself as having unlimited power, like a medieval monarch, the constitution does not permit him "to do anything he wants," reports The Guardian.

"I'll just give you an example that shows you the difference between him and a king. The constitution says there can be no titles of nobility. So while the president can name his son 'Barron', he can't make him a baron," Karlan quipped drawing chuckles in the congressional hearing room.

The Stanford Law Collage professor was addressing the lawmakers during the House Judiciary Committee's first hearing on impeachment, which featured three other constitutional scholars- Harvard law professor Noah Feldman; University of North Carolina law professor Michael Gerhardt; and George Washington University law professor Jonathan Turley. While Jonathan Turley was called to argue for the panel's Republican members, the other three were there to testify by the committee's Democrats.

However, the pun didn't go down well with the first lady, who argued it was no laughing matter. Shortly after Karlan's address, the 49-year-old tweeted: "A minor child deserves privacy and should be kept out of politics."

"Pamela Karlan, you should be ashamed of your very angry and obviously biased public pandering, and using a child to do it," she further wrote in her tweet. POTUS also retweeted his wife's message.

Barron Trump, Donald Trump, Melania Trump
US President Donald Trump, First Lady Melania Trump and their son Barron arrive for a new year's party at Trump's Mar-a-Lago resort in Palm Beach, Florida, on 31 December, 2017 NICHOLAS KAMM/AFP/Getty Images

During the impeachment hearing, when Karlan clashed with Republican congressman Matt Gaetz over her donations to Democratic politicians, the latter also targeted her for making a joke about the 13-year-old and said invoking Trump's son "does not lend credibility to your argument. It makes you look mean."

Karnal later expressed regret on her comment and told the panel: "I want to apologise for what I said earlier about the president's son. "It was wrong of me to do that."