The radio icon laid into the Brooklyn Nets guard in an epic rant on his SiriusXM show ... explaining he cannot wrap his head around Irving's refusal to get -- and promote -- the shots.

"In terms of idiots, he's got to be the top idiot in the country right now," Stern said of Irving. "Guy's got a chance as a young man to make millions of dollars, all he's got to do is get vaccinated but he doesn't want to get vaccinated."

Irving was just barred from both Nets practices and games last week because he won't get the vaccine ... and in a rambling social media video shortly after the announcement, he said he has no plans as of now to change that -- even if it cost him MILLIONS in lost paychecks.

Stern said that's one of the dumbest things he's ever heard ... explaining he believes Irving is straight-up "stupid."

"This guy's got a history of being stupid," Stern said. "He's one of those guys who was a flat-earther too. And, you know what, he doesn't know anything. His mind works in a very strange way."

Stern then praised the Nets for their firm stance in keeping Irving away from the team amid his vaccine decision, further ripping into the NBA player.

"I don't know who runs the Brooklyn Nets, but I wish they were running the country," Stern said. "I love this. Basically saying, 'Listen, douchebag. You've got to do what is right. That's it.'"

Irving has previously stated he's NOT against getting the vaccine ... but claims his stance is to shine a light on those losing jobs over strict mandates forcing people to get the jab.