President Donald Trump called former Secretary of State Rex Tillerson "dumb as a rock" and "lazy as hell" in an insult-laden tweet Friday.

The tweet came in response to Tillerson on Thursday calling Trump "undisciplined" and saying the president frequently asked him to break the law.

Tillerson left the administration in March 2018 after a rocky tenure, during which he frequently clashed with Trump.

President Donald Trump lashed out at former Secretary of State Rex Tillerson in an insult-laden tweet after Tillerson called Trump "undisciplined" — and said the president frequently asked him to break the law — while speaking to a crowd in Houston Thursday night.

"Mike Pompeo is doing a great job, I am very proud of him. His predecessor, Rex Tillerson, didn’t have the mental capacity needed," Trump wrote.

"He was dumb as a rock and I couldn’t get rid of him fast enough," Trump added. "He was lazy as hell. Now it is a whole new ballgame, great spirit at State!"

"Dumb as a rock" appears to be one of Trump's preferred insults, as BuzzFeed reporter Vera Bergengruen suggested on Twitter. He's used the term to describe former Florida Gov. Jeb Bush, "Morning Joe" host Mika Brzezinski, and the media commentators Jonah Goldberg, Rick Wilson, Glenn Beck, and Bill Maher.

Tillerson, who served as ExxonMobil's CEO before joining the Trump administration, has maintained a relatively low public profile since leaving the State Department in March this year.

"When the President would say, 'Here's what I want to do and here's how I want to do it,' I'd have to say to him, 'Well Mr. President, I understand what you want to do, but you can't do it that way. It violates the law. It violates a treaty," Tillerson told the attendees at a fundraiser for the MD Anderson Cancer Center in Houston.

Tillerson also described it as a "challenge" to move from the "disciplined, highly process-oriented" culture at Exxon to work for someone "who is pretty undisciplined, doesn't like to read, doesn't read briefing reports, doesn't like to get into the details of a lot of things."

Read more: 'Just answer the question and kill this story': In internal emails, Heather Nauert criticized Rex Tillerson's refusal to deny reports that he called Trump a 'moron'

Trump struck a remarkably different tone on Twitter from he did when he first hired Tillerson in December 2016. At the time, Trump described Tillerson as "a world class player" who made "massive deals" in countries including Russia.

Their relationship soured considerably over disagreements on foreign-policy issues, and NBC reported that Tillerson called Trump "a f------ moron" after a 2016 meeting. While Trump dismissed the story as fake news, Tillerson never denied making those remarks.