The True Lies star thought it would be fun to step into the gym, shed a few pounds and toughen up - but he admits he wasn't looking forward to getting into the ring with his spiritual adviser for the charity scuffle, dubbed the Rumble in the Temple.
He says, "It's a lose lose... You either lose to your rabbi or you beat up your rabbi."
The Jewish punch-up raised cash for a rehab centre, and Arnold was surprised so many people wanted to see someone smack him in the face.
Arnold was trained by best pal Dax Shepard for the fight, while his rabbi asked top boxing guru Freddie Roach to help him prepare for the clash - and that scared the star into training hard.
The actor adds, "I thought maybe he gave the rabbi some kind of secret thing and they were going to lay it on me, so I didn't want to take any chances."