shit a six
A huge shit. (Origin is from when you take a really big dump and in curls up in the toliet into a 6.)
"Hey guys i'll be right back, i'm about to go shit a six."
shit over a six rail fence
This would mean you have to shit really bad. So bad that you could shit over the top rail of a six rail fence which would be 9 feet or so.
Man after eating lunch from the mexican roach coach I fell like I could shit over a six rail fence .