My first job out of the Army was in New York City; a lot of new buildings were going up in Manhattan. In an attempt to beautify the city, city codes of the time encouraged architects to set the buildings back from the sidewalk, creating a small plaza, and add some "art". There were some property tax breaks to prompt this.
Always looking to cut costs, some of the builders simply had very large, irregular shaped boulders trucked in and plopped down in the plaza. There, that's the art.
The Architectural Critic of the New York Times, reviewing the opening of a newly created building, coined the phrase when he opined (don't remember his exact words), "Sadly, it is another example of turds in the plaza passed off as art."
Rusty