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."
Shitfishing is a word that simply means the irrational attempt to secure some anal intercourse. This can be with a current girlfriend or in pubs and clubs. To be a shitfisher is often a hit and miss experience with some girls revolted by the idea. Certain females are known to warm to the idea (no pun intended) and a good shitfisher, if indeed that is possible, will hone in on these ladies.
A successful shitfisher is often heard to yell "i've got a bite" midway through performing the act he has been longing for through out the night.
Mark: "hey, are we going to the clubs to pick up women yeah"
Juan: "only if you come shitfishing with me"
Mark: "but i only catch obese fish"
Juan: "more to fill your plate with *intolerable wink*"