When somone puts their arm down the piping of the toilet to find a shit. This is usually performed after somone else has just been to the toilet, obviously increasing the chance to find something.
There are many reasons people do shit fishing, but the three most commen reasons are:
Somone ate something that the person needed
"Hey John, wanna play some b-ball on saturday?"
"I can't Stan, I'm going Shit Fishing with my dad."