To expel raw sewage into ones pants due to surprise, shock or disgust.
Surprise: When I realized Clarence was a girl, I almost dumped my trunks.

Disgust: When that new Avril Lavigne song came out on the radio, I pretty much dumped my trunks.

"Oh crap. I'm going to dump your trunks."
by Bobby Bobbinson May 3, 2008