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PITTSBURGH, Pa. — Billionaire businessman Mark Cuban officially endorsed Hillary Clinton at a surprise appearance during a rally here Saturday night, where he also railed against Donald Trump.

Cuban, who first said he would vote for Clinton via tweet Saturday, opened his remarks by saying "hello” to Trump in Russian. He then bragged that 'Shark Tank,' a television show he invested in early on, "kicked 'The Apprentice's' ass."

During his introduction of Clinton and her running mate, Tim Kaine, Cuban went after Trump and got a huge reaction from the crowd of 5,100 when he used a Pittsburgh slang term that means someone who is inept to describe the Republican nominee.

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(L-R) Anne Holton, democratic vice presidential nominee U.S. Sen Tim Kaine (D-VA), democratic presidential nominee former Secretary of State Hillary Clinton, Mark Cuban, U.S. Rep. Mike Doyle (D-PA) and former U.S. president Bill Clinton raise arms during a campaign rally with at the David L. Lawrence Convention Center on July 30, 2016 in Pittsburgh, Pennsylvania. Justin Sullivan / Getty Images

"You know what we call a person like that, you know, the screamers, the yellers, the people who try to intimidate you? You know what we call a person like that in Pittsburgh? A jag-off. Is there any bigger jag-off in the world than Donald Trump?" Cuban said.

The Pittsburgh native later told CNN that he decided to support Clinton because Trump “scares” him.

"Initially, I really hoped he would be something different, that as a businessperson, I thought there was an opportunity there. But then he went off the reservation and went bats--- crazy, Cuban said.

He also mentioned that he had met with Trump in recent months and told him "at some point you have to learn, at some point you have to read."

A Clinton aide said that Cuban had been in touch with their campaign about an endorsement for a few weeks. Cuban, who is friendly with former President Bill Clinton, had dinner with him earlier this month and also visited Brooklyn headquarters recently, the aide added.

Cuban called campaign chairman John Podesta on Thursday, before Clinton delivered her speech at the Democratic National Convention. He told him that he was "ready to go" and wanted to go ahead with a formal endorsement.

Mark Cuban gives the thumbs-up before the start of a campaign rally with democratic presidential nominee former Secretary of State Hillary Clinton and democratic vice presidential nominee U.S. Sen Tim Kaine (D-VA) at the David L. Lawrence Convention Center on July 30, 2016 in Pittsburgh, Pennsylvania. Justin Sullivan / Getty Images

On Saturday, when Clinton took stage, she thanked Cuban for his support and said he "exemplifies" the American dream because "he understands the dream gets bigger the more you share it."

Kaine, for his part, joked that “one of the hard things” about being selected as a vice presidential candidate is that "they make you follow Mark Cuban."

Clinton and Kaine are in the midst of a three-day, jobs-focused bus tour through Pennsylvania and Ohio.