My informant first heard this folk simile as a child growing up on a farm in Nebraska. One day when he was out with his father, it began to rain. While rain was not out of the ordinary at that time of year, the rain was coming down with unusual ferocity. My informant recalled that the wind was blowing the rain in every which direction and when the rain hit the ground, it splattered everywhere. Another farmer turned to my informant’s father and rattled off this folk simile.
Growing up on a farm, my informant knew from experience exactly what happens when a cow pisses on a flat rock. “It’s splatters everywhere and makes a huge mess,” he explained. This is not a secret, and anyone can understand how this directly compares with a heavy rainstorm. But for one to fully appreciate the humor in this simile, they would have to have a first-hand experience to relate to. For this reason, this folk simile is mostly shared among farmers and others residing in rural communities.
There’s no underlying message that can be found within this simile. It’s used because it takes something that’s funny to think about, to the folk group, and applies it to an unfavorable situation. It turns an unfavorable rain storm into something to laugh about.