Beat the shit out of him with an indicator stick, he’ll never be able to describe the weapon to the police.
Explanation: This joke plays on the stereotype that BMW drivers often don’t use their turn signals (indicators) while driving. The punchline humorously suggests that if you hit a BMW driver with an indicator stick, they wouldn’t know what it is or be able to describe it to the police, since they supposedly never use it. The humor comes from this exaggerated and playful stereotype.