Kenyon Martin is UNLOADING on his former Denver Nuggets coach George Karl -- saying the guy is a "terrible person," "awful coach" and a "coward."

Martin is clearly furious with Carl over a new tell-all book the coach is about to drop -- in which Karl rips Kenyon as a "spoiled brat" and Carmelo Anthony as a famewhore and a "user of people."

Martin's immediate reaction -- "The Nerve of an AWFUL AND COWARD ASS COACH."

He continued, "Everyone that's played for that awful person and coach can't stand the ground he walks on."

"I didn't have a father going up. We all know that. What's George Karl excuse for being a terrible person."

There's more ... 

"Since we were so bad as players. Tell him to forfeit those Denver wins. Having a lot of wins doesn't make you a good coach. He was blessed with great talent."

We're guessing this ain't done yet. Stay tuned ...