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It seemed like a joke, but Barstool Sports president Dave Portnoy tells TMZ Sports the organization REALLY did fire Michael Rapaport this weekend after he insulted the company's fans.

It all started when Rap posted a tweet saying "Stoolies" are losers in life. Portnoy says he and the rest of the staff took offense ... "That was the ultimate deal-breaker."

Portnoy says Rapaport was under contract with Barstool -- as part of the deal, he would create original content like viral videos and Barstool would "rep" Michael's popular podcast, "I Am Rapaport."

It seemed the two sides had a great business relationship -- but Portnoy says it turned into an "utter and complete train wreck."

Portnoy tried to fire Rapaport over text message -- but accidentally sent it to NFL insider Ian Rapaport instead. Eventually, Michael got the message.

"To me, it was very personal. It was very real," Portnoy says.