Jaylen Brown is paying the price for how he celebrated his insane dunk over Detroit Pistons forward Isaiah Stewart ... with the league hitting him with a hefty fine over it all.
The play in question went down with 12 seconds to go during the Celtics' win over Motor City on Wednesday night, with Brown hitting a nasty poster on Stewart to get the TD Garden going crazy.
In the moment, the 28-year-old emphasized the highlight by making a throat-slashing gesture toward Stewart -- and it turns out the NBA wasn't a fan of the move .