Rob Gronkowski -- Vegas WRESTLING with Broken Forearm

Rob Gronkowski scared the hell out of New England Patriots fans last night ... when the superstar tight end BODY SLAMMED his friend on stage at a Las Vegas nightclub ... landing hard on his broken forearm ... and TMZ has the footage.

Gronkowski -- who signed a $54 million six-year contract extension in June -- broke his forearm last season, then injured it again during the Pats' first playoff game against the Texans.

But that didn't stop him from executing a Stone Cold Stunner-esque move on a friend during a playful dance battle at XS nightclub last night.

As Gronkowski was throwing his friend to the ground, the tight end -- who was wearing a protective sleeve on his busted wing -- also crashed to the ground and smacked his damaged arm on the stage in the process.

Gronk popped right up and waved his arms around -- a sign that he didn't reinjure anything during the stunt -- but seriously, HOW DUMB WAS THAT?

In fact, we gotta ask ...