Today it is possible to get a map of essentially anywhere on Earth, but in years gone by there were plenty of empty holes in even the best world maps. A favourite way of indicating that these gaps were unknown and potentially dangerous was to mark “Here be Dragons” on the map (unless it was a Pirate Treasure Map, which usually had “Here be Sharks” instead).
An OpenStreetMap contributor recently moved out of London to the city of Durham, which has not yet been mapped in the OSM database. He has decided not to use any map that he can’t legally modify and display however he likes, which basically leaves him with the dragons. When friends invite him to a certain location, they need to sketch a map rather than simply give an address to look up. That is, until he has added that location to the OpenStreetMap.