Kyrie Irving and Celtics fans are still beefin' ... the Nets star point guard gave double middle fingers behind his head to a group of fans who were heckling him during Sunday's game.
Celtics fans were all over Kyrie -- who spent two seasons with Boston before signing with the Nets -- trying to get him off his game ... and Kyrie seemingly had enough and responded.
In the short clip, you see Irving flip not one but two birdies behind his head as he stands along baseline, waiting to receive the ball from the ref before inbounding the ball.