slobber = foaming at the mouth
knocker = punching each other out

The first thing that came to my mind when I saw this phrase (and I had never heard it before) was a boxing match, specifically those slo-mo scenes from Rocky (and every other boxing movie) where after a particularly good whack to the head you can see a prodigious flume of spit 'n' blood shoot out of the punchee's mouth. The other guy has knocked the slobber out of him.

Lllllllladies and gentlemen, lllllllllllet's get rrrrrrrrrrrrready to knock ssssssssssslobber!