When you take a leak in an urban environment. E.g. in a telephone box, in a doorway or on a car. Usually coincides with alcohol consumption. As your level of intoxication increases, the locations of your urban pisses become more and more antisocial.

Sometimes shortened to just 'urban', when used in a context where the meaning is obvious.

When you eat dirt for the first time and leave your family to build a shack in the woods