On the 8th of August 2004, a tour bus belonging to Dave Matthews Band dumped about 360 kilograms(800 pounds) of human shit onto a sightseeing boat in the river. They dumped the feces from the Kinzie Street Bridge in Chicago giving the incident it's name.
An idiom used to explain how a seemingly negligible or "weightless" variable can have a massive, tangible impact on an outcome.
It refers to the physics concept of radiation pressure. Because light carries momentum, the sunlight hitting a city like Chicago on a clear day actually pushes down on it, adding roughly 300 lbs of physical force to the city's total weight.
Example 1:
Person A: "I only changed a single line of code and the entire server crashed. I don't get it."
Person B: "Chicago weighs more on a sunny day!"
Example 2:
Person A: "I barely touched the gas pedal and we spun out!"
Person B: "Chicago weighs more on a sunny day, man."
Example 3:
Him: "She broke up with me just because I didn't hold the door open?"
Her: "Chicago weighs more on a sunny day."
Example 4:
"If I had found my keys ten seconds faster, I'd be here. Chicago weighs more on a sunny day."