UPDATE, April 6: In the season finale, it is revealed that a Chicago Sunroof is defecating through the open sunroof of a parked car.
With just three episodes, AMC's Better Call Saul has become the best show on television. I was going to add "arguably" in there, but thought better. It is the best show. It adds several dollops of humor and art-film techniques to Breaking Bad's tight plotting and moral skating. Naperville native Bob Odenkirk, one of the great Chicagoans to have gone through the SNL factory, has always excelled as a nervous man about to blow his gasket, but now he gets emotional buoyancy and sneaky intelligence under his character. Last's night episode, "Nacho," opened in a flashback, with Odenkirk's greasy con man Jimmy McGill locked up in Cook County Jail. He is pleading to his older brother (Michael McKean), a lawyer of some skill, to help him out.
Now, in the scene, the older McGill warns his little brother that the charges against him could brand him a sex offender. To which Jimmy explains that all he pulled off was a simple "Chicago sunroof." So what the hell is a Chicago sunroof?
Earlier in the season, Jimmy tells of his past as "Slippin' Jimmy," detailing his familiar con of falling on icy State Street sidewalks for hush money by threatening to sue businesses. So a Chicago sunroof is likely a con. Though it is just as likely completely made-up and undefined, an act for us to imagine. Definitions quickly appeared on Urban Dictionary. Users "dirty beaner" and "FupaGunt" naturally attempted to turn it into a weird sex act or prank, because Urban Dictionary is largely edited by college freshmen. Considering the charges against the soon-to-be-Saul include destruction of property and assault, farting on someone's head seems unlikely.
The more colorful definition of "Placing your penis on the head of a unsuspecting person" has more juice, but I can't see the show trying to make the protagonist sympathetic by slapping his johnson on people's skulls. Also, that does not involve property damage.
Paste Magazine came up with some elaborate speculations involving taxi cabs and decapitation.
It's most likely a scam gone wrong. Here's my guess:
Jimmy rents a hotel room. He drills a hole in the ceiling, creating a peep hole. He waits for someone to be in the room above. He then plays a realistic recording of people having sex (remember, in this same episode we learn he's known for his "robot sex voice"). He waits for someone to peep through the hole at what is going on. Then, he races to hotel management and threatens to sue. They get into a heated argument and punches are thrown. Assault, property damages, sex offender. Possibly.
So, what do you think a Chicago sunroof could be?