Named for the address of the original website, (goatse.cx) which has since been taken down but not before achieving Internet noteriety. See also tubgirl
The website featured a man bending over with two hands stretching apart the largest bodily orifice in the world. You could see inside several inches deep.
Since then, parody websites have sprung up showing everything from stretched-apart doorways to red-filling-ed cookies.
"Heh. No really, Aunt Sally. I swear goatse.cx has the best apple pie recipes on the Internet."
"Oh god! The Grand Canyon looks just like Goatse! No burro ride for me, thanks!"