Snap a log 

When one is defecating and proceeds to inadvertently sever their waste usually causing a high degree of cleanup to be required, normally involving a large amount of toilet paper.

Person 1: “Don't take too long in the bathroom, man.”
Person 2: “I'll try not to. Wouldn't want to snap a log.”

Snap a log by trentsteele April 30, 2014

snap off a log 

Taking a dump, in particular a long stiff tubular type that holds its shape well.

Man, I gotta snap off a log!

snap off a log by Turd Burger Boy October 6, 2003