John Kerry was not remotely afraid to lay into President Donald Trump in an interview on Friday.

Speaking on HBO’s Real Time with Bill Maher, the former secretary of state and Democratic presidential candidate said he believes Trump has “the maturity of an 8-year-old boy with the insecurity of a teenage girl.”

“That’s just who he is,” Kerry added of the president.

Bill Maher asked Kerry in the interview why he believed Trump was mad at him, and Kerry answered, “I don’t know. I think I told the truth.”

On Thursday, the president tweeted: “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!”

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“Everybody does,” Kerry said about holding such “illegal meetings.” “Henry Kissinger [another former Secretary of State] for 40 years has been traveling to Russia, traveling to China, talks with the leaders. There’s absolutely nothing usual about it. The conversation I think he really ought to be worrying about is Paul Manafort with [special counsel Robert] Mueller.

Manafort, one of Trump’s former campaign managers in 2016, accepted a plea deal on Friday regarding his work as a political lobbyist in Ukraine over the last 12 years. Manafort has thus pledged to cooperate with Mueller “fully and truthfully” on the investigation into Russian meddling in the 2016 election.

Kerry continued blasting Trump for his continued lies and deception. The former secretary of state added that he believes the public revelations that have arisen as a result of the president’s behavior, including Bob Woodward’s tell-all book Fear and the New York Times anonymous op-ed article, are unlike anything he has ever seen.

“It really is giant, and soon you will be hearing him say, ‘That’s the worst deal that’s ever been made,’” Kerry added. “When you stop and think about it, it’s gone from the ‘art of the deal’ to the ‘art of the squeal.’ I, honestly, have never seen anything quite like it: the anonymous op-ed, the Woodward book, the chronology of everything that’s been happening. And it’s tough.”

Kerry’s latest memoir Every Day Is Extra was published on Sept. 4.