**WARNING -- THERE IS A LOT OF CUSSING IN THIS VIDEO**

It's the breast fight you're gonna see all week ... a bench-clearing RUMBLE at a Legends Football League game -- which culminated in a linebacker SOCKING an opposing coach!!!

It all went down in Vegas where the Atlanta Steam defeated the Jacksonville Breeze 20 to 12 in an overtime thriller.

But things hit a boiling went when one of the Jacksonville players got into it with a member of the Steam -- and called her a "p***y a** b**ch."

That's when the benches cleared ... the fists began to fly ... and Jacksonville's Adrian Purnell punched Atlanta Steam head coach Dane Robinson right in the face.

Security jumped in an tried to break it up -- but the women were out for blood.

It appears one of the women even swings her helmet at an opposing player -- which would almost certainly warrant an automatic suspension if it happened in the NFL.