A Slumdog Millionaire is when you are having sex with a girl in the ass and when you are about to cum, you pull out and cum in her purse.
It is typically best practice to exit immediately. It is also recommended on one night stands, not with a current girlfriend or wife for obvious reasons.
Matt ended up hooking up with a girl after a night at the bar. He convinced her that they should have anal sex. After about 5 minutes of anal pleasures, Matt realized he was getting close to climax. He knew he was never going to see this girl again and decided to pull a Slumdog Millionaire on her so that he could brag about to his friends. He proceeded to pull out and jiz in her purse that was conveniently located next to her bed.
Matt immediately grabbed his clothes and ran out of her room.