u(you) r(are) p(ulling) l(legs); it means that you have been accused in a chat or other exchange of making fun, making things up, or otherwise being whimsical. In some circles "urpl" is considered "fighting words". Mostly these circles are composed of crabs (who, appropriately, do not like having thier legs pulled, off or otherwise. TLDR; crabs can't take a joke.

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