LeBron went off on his latest episode of "The Shop," after being asked why he hates Boston. His blunt answer ... "Cause they racist as f***."

King James went on ... "They will say anything. And it's fine. It's my life, f*** I've been dealing with it my whole life. I don't mind it. I hear it. If I hear somebody close by, I check them real quick, then move onto the game. They're going to say whatever the f*** they want to say."

James offered receipts for his conclusion ... reminding folks about the 2012 incident at the Eastern Conference Finals, where Celtics diehards doused him with a drink as he walked toward the locker room.

LeBron's not alone. Other players have also groused about the way Celtics fans behave, Klay Thompson complained, "We've played in front of rude fans before. Dropping f-bombs with children in the crowd. Really classy. Good job, Boston."

Draymond Green's wife blasted them for chanting "f*** you Draymond" during the NBA finals.