Q:

Is "Chicago sunroof" a real expression?

The expression is from "Better Call Saul". It was defined in the season finale:

defecating into a car through an open sunroof as a prank

So far, the only resource I've found that corroborates this is Urban Dictionary, which isn't very reassuring.
Did this expression exist before the show? Is this a common American expression? For that matter, is this a common prank? And what is it called, if not "a simple Chicago sunroof"?

A:

It appears it is just related to the show, as of now. 
From: ‘Better Call Saul’ Creator Peter Gould  Explains The Origin Of The ‘Chicago Sunroof’

A great mystery of Better Call Saul‘s first season was what, exactly, a “Chicago Sunroof” was. All we knew through nine episodes was (A) that Jimmy McGill had done it to someone back in his Illinois Slippin’ Jimmy days, and (B) it resulted in criminal charges so sticky that Chuck had to swoop in from New Mexico to save the day. Then came the finale. And the reveal. During what can only be referred to as a full-on bingo meltdown. And it turns out that a Chicago Sunroof is when you defecate through someone’s sunroof. To quote Jimmy, “It’s a real thing. I didn’t make it up. I’m not the first person to do it. There’s a name for it.”
But is it, though? Is it a real thing? Esquire caught up with Better Call Saul creator Peter Gould to get the full Chicago Sunroof story.

“We made it up,” Gould says. “In the writers’ room we were talking about Jimmy and his ups and downs. We talked about what he would get arrested for and we had this image of him getting drunk and taking revenge on some enemy by defecating through his sunroof. Because we knew he was living in Cicero, we dubbed it the Chicago sunroof.”

So, there you have it. A Chicago Sunroof is not a real thing. Or rather, it wasn’t a real thing. Because thanks to Peter Gould and the Better Call Saul writers, it is very much a real thing now. Close your sunroofs, people.

