It means that it has already been determined. It cannot be changed. This is from the bible. It is written in the book of Daniel from the bible that a hand appeared and wrote in front of king Belshazzar of Babylon a message saying that he will be dethroned and his kingdom divided. It was a message from God and can not be changed. It was going to happen. That is why it has its meanimg.

"The writing is already on the wall"= the result is already determined or known Example: "The writing is already on the wall for those who smuggle drugs into Indonesia"