Urban

Urban is from the Greek word αρχαίος απαγόρευση.

Definition: αρχαίος - ancient, and απαγόρευση - ban/blocking/obstacle.
The total meaning is old/ancient ban.

"This dictionary is urban. That's why it is called "Urban Dictionary". It is banned by the government and it has been banned for a long time, so therefore it is an old ban."