Tomlin 'Handed Me An Envelope'

James Harrison says Pittsburgh Steelers head coach Mike Tomlin "handed me an envelope" after one of the most vicious hits of his career ... calling it the "G-est thing" Tomlin ever did.

The hit Harrison is referring to went down against the Cleveland Browns in 2010 ... when he led with his helmet and laid out a defenseless Mohamed Massaquoi.

The hit resulted in a $75,000 fine and Massaquoi left the game with a concussion ... and now, Harrison is sharing his side of the infamous incident.

"Listen, on everything I love, on my daddy’s grave, I hit that man with about 50% of what I had and I just hit him because I wanted him to let loose of the ball," Harrison told Barstool Sports' "Going Deep"…