A combination of the two most pleasurable feelings. A good shit. And a good wank.
A shank, by definition, is when the two above sensations culminate in a single toilet sitting. In this scenario, toilet paper serves a dual purpose. The ultimate, or true definition of a shank is when climax and release of feces is simultaneous.
I saved time this morning by having a shank.
"Dude, did you go to the toilet and have a shank? You look relieved!"
"That was amazing, I just had a shank! I love having a number 2 and a number 3 at the same time!"