Stretching an anus so much that it becomes a wide, gaping hole. When done successfully it will resemble a donut hole. It can by done with the help of a man's genitals, a sex toy, a fist, and/or any other object.
Steve: "Hey guys, let's pin down Jessica and bang her butt and then look at her open, stretched hole!"
Bobby: "What for?"
Steve: "Because she told me she wants to join the donut club!"