One piece is the enormous kingdom of a country that does not exist anymore.
In the manga by robin's old teacher he is said: "But the most essential thing that holds the key to everything is the name of the once prosperous kingdom"...and then is cut off.
"Any information regarding this country had been entirely erased."
Just imagine in HYPHOTHETICAL terms China overtakes the whole world and tries to censor everything like they are doing with tinamen square. Everyone born after the supposed event of china overtaking the world have no PRIOR knowledge of what existed before simply because all that knowledge is wiped out. Then there are the pirates and they are all out searching for this supposed loot called the One Piece. One piece in japanese means a combined mass that makes a treasure. Why do u think the world government would be so mad of pirates looking for loot. ONLY if its the country that they overtook and are trying to conceal from the whole mass public
In the recent chapter of whitebeards death: http://read.mangashare.com/One-Piece...6/page012.html
From far away and long ago, it has continually been passed down time and time again. and one day, hundreds of years into the future ,all of it will be written in history , those who came to fight for this world(which probably means the once ultimate kingdom who was a really great world and was OVERTAKEN by the world government)
its going to come, a grand battle that engulfs the entire world is comming.
ONE piece it exists!"
The only time when everyone would get up in arms even civilians are if there was a hidden history in which everyone once lived in a prosperous place but was taken from them and later concealed by the world government.
That is a very good piece in which it tells us something people dont think exists, exists. Which i know is common sense, but just imagine in the one piece world, everyone except the high ranking gousei and sengoku dont know of the void century and of the once prosperous kingdom overtaken by the world government which is highly sensitive information. But the idea of the One piece(once prosperous kingdrom) existing is what the world government is VERY VERY keen on, just look at sengokus face.