Celtics star Jaylen Brown is officially the John Wick of the NBA ... because's he's vowing REVENGE on a fellow association player over a heinous popcorning incident from his rookie year.

We got Brown out in NYC and asked him a very simple question ... what's the best locker room prank he's seen in his young NBA career?

That's when JB revealed he's a victim ... telling us his car was filled up with popcorn his rookie year by veteran Amir Johnson, and then promising to deliver some get back for the transgression.

"Amir Johnson, I'm coming for you."

Sleep with one eye open, Amir.