A leftover nugget of excrement in the rectum that makes itself known after one has already left the house, and is too large for gas to pass around without soiling one's self. The gas builds until the subject is able to find a toilet, and said excrement is propelled out with considerable force as if shot from a cannon. This usually results in a soaking wet ass.
I'm terrified of using public toilets, so I've been fighting this rogue cannonball all night. When I get home and fire this thing I might crack the bowl.