Fat Joe got a bunch of kids to lean back and crank the fun all the way up ... at a bar mitzvah!
The rapper made the surprise appearance in NYC Friday night for a boy named Casey, son of Ira Judelson -- bail bondsman to the stars.
Joe showed up at 7:15 PM and stayed for almost 2 hours ... performing "All The Way Up" and "Lean Back."
Fat Joe is business partners with the kid's dad. Who knew bail bonding was so baller?