The eruption smelly gas coming out of the anus. Sometimes it makes a noise sometimes it doesn't. The one that makes a noise is known as a trouser sneeze. You have to be careful not to follow through. See fart, trump and air biscuit.
"I've just done a trouser sneeze lads"
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Ejaculation in one's own pants, literally. Metaphorically used as an expression of ecstatic excitement.
"Did you just snow your dacks?"
"You bet I did! I just had a massive trouser sneeze"