Once used to describe an object that grasps like a tail, this is now used much like the word "Smuf" (as a noun, verb, adjective and so on). It is also commonly used as a substitute word in colloquial phrases.
one who cannot poop in a toilet in a reasonable manner may be said to have a prehensile rectum. A prehensile appendage is one that is capable of grasping or conscious manipulation. One with a Prehensile rectum is purposefully able to miss a large target such as a toilet.
the term originates from a post by Billy Rubin on the StraightDope forums in 2001.
Look, F***wicket,I know you got your problems. Do you have a prehensile rectum that you can extend on a stalk like a snail's eye and aim? How in the name of all that is unholy do you get poop everywhere?