A fecal stain that is visible on one's undergarments.
Also presents in a less structural (and lesser known) form, typically developed from excessive "dry farting". In this form, the skidmark develops in a larger region, yet its definition is not as clear. Essentially, it is representative of a consistent gaseous pounding of one's underpants over a long period of time, and while fairly rare, is unmistakable. Usually, this effect is understandable to about 1/10th of all humans.
I just ran a red light and blasted a massive skid mark all over my BVD's.
Essentially the reverse of the venerated "Shocker", the "Astonisher" is achieved simply by putting one in the pink and two in the stink. While lesser known, the Astonisher has actually been scientifically proven to be more effective than the Shocker.
When doing double penetration, the Astonisher is the preferred method.