skid marks
Brown Y-fronts invariably had skid marks (actual evidence was unneccesary for conviction) as did any undergarment with a even a suggestion of yellow piping. Strong correlations were found between those sporting lobsters at swimming practive and the presence of skid marks thus causing prolonged embarrassment. Further etymological and historical research can be undertaken if necessary.
written by Be* , approved by Log

We actually had a game called 'Skids' at our (no surprise) all-boys school. The aim was to produce the most impressive skid marks in your pants. Anyone actually shitting themselves would immediately lose...
...though now that I come to write this down, it's quite clear that we were all losers. Losers with shitty underwear.
written by an*ny*ou* u*er, approved by Ponky