The phrase may have originated in Scotland.

It is first found as "Possession is eleven points of the law" in 1629 in Remaines concerning Britaine.

http://english.stackexchange.com/questions/36390/where-does-the-phrase-possession-is-nine-points-nine-tenths-of-the-law-c

It became "nine points of the law" by the 1670s. "Nine parts of the law", first appeared in 1785. "Nine tenths of the law", in 1814.

The modern use of the saying is to sort of mean, "finders keepers". Or that possessing something is powerful evidence that it is yours, which assumption can only be overturned by clear contrary evidence.

The original meaning may have been more mundane and descriptive, just indicating that most (11 out of 12) laws are to do with property.