A phrase used in reference to a person with attractive, round buttocks; preferably large ones. Commonly used in reference to such type persons who may also have a less attractive trait in comparison.
Derived from an infamous thread on 4chan where someone desperately pleaded for pornographic pictures, showing Arthur and Sue Ellen from the PBS kids cartoon 'Arthur' having sex. The said person repeatedly used the mentioned phrase to describe Sue Ellen's buttocks.

When you eat dirt for the first time and leave your family to build a shack in the woods