Former Secretary of State John Kerry John Forbes KerryThe Memo: Warning signs flash for Trump on debates Divided country, divided church TV ads favored Biden 2-1 in past month MORE mocked President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE on Friday night, saying he has the “insecurity of a teenage girl.”

Kerry opened his appearance on HBO’s “Real Time with Bill Maher” by hitting Trump's frequent use of social media and touting former Trump campaign chairman Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE's plea deal with the special counsel's office on Friday.

The 2004 Democratic presidential nominee and former Massachusetts senator then launched into a personal critique of Trump.

"I don’t want to get into a real riff on this but I’ve got to tell you," Kerry began.

"Do it!" an audience member is heard yelling.

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Kerry smiled and gestured out toward the audience before finishing his thought about Trump.

“He really is the rare combination of an 8-year-old boy — he’s got the immaturity of an 8-year-old boy with the insecurity of a teenage girl,” he said.

During his appearance on the show, Kerry pushed back on Trump's Twitter attack from the previous night when the president accused him of illegally meeting with foreign dignitaries after leaving the State Department.

“John Kerry had illegal meetings with the very hostile Iranian Regime, which can only serve to undercut our great work to the detriment of the American people. He told them to wait out the Trump Administration! Was he registered under the Foreign Agents Registration Act? BAD!” Trump tweeted on Thursday.

Maher asked Kerry on Friday what he had done that was so “bad” in Trump's view.

“I think I told the truth,” Kerry said, to cheers from the studio audience.

Kerry, who helped lead the Obama administration negotiations for the multination nuclear deal reached with Iran in 2015, insisted that there is “absolutely nothing unusual” about ex-diplomats continuing to meet with officials from other countries.

The former diplomat argued that Trump should be more worried about Manafort's plea deal with Mueller, which includes fully cooperating with the probe into Russian meddling in the 2016 election, instead of worrying about Kerry's conversations with Iranians.

“It really is giant, and soon you’ll be hearing him say that is the worst that’s ever been made. When you think about it, it’s really gone from the 'Art of the Deal' to the art of the squeal,” Kerry said.