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A public bus (usually an intercity route) that has no bathroom and that goes for a significant amount of time (say an hour or longer, sometimes several hours) without making any stops to allow passengers to use the restroom.

These buses are called pooping buses because not everyone can hold it in for the long ride, leading some passengers to go in their pants.
When riding on a pooping bus, wearing a diaper is recommended.
by Bed time June 21, 2010