Wiktionary
prov. One who has possession of a thing has some right to it; (non-gloss definition: a popular statement of the doctrine of adverse possession.)
Wikipedia
Possession is nine-tenths of the law is an expression meaning that ownership is easier to maintain if one has possession of something, or difficult to enforce if one does not. The expression is also stated as "possession is nine points of the law", which is credited as derived from the Scottish expression "possession is eleven points in the law, and they say there are but twelve."
Although the principle is an oversimplification, it can be restated as: "In a property dispute (whether real or personal), in the absence of clear and compelling testimony or documentation to the contrary, the person in actual, custodial possession of the property is presumed to be the rightful owner. The rightful owner shall have their possession returned to them; if taken or used. The shirt or blouse you are currently wearing is presumed to be yours, unless someone can prove that it is not."
Usage examples of "possession is nine-tenths of the law".
When they discovered that Tia had already tied the ship and its contents up legally on Hank's behalf, they moved on the principle that 'possession is nine-tenths of the law, and the fellow arguing the other tenth has to prove it with a lawyer'.
Her Majesty's Government takes a different view, and as they say, possession is nine-tenths of the law, but the provisions of the Act of Annexation specifically delimit my powers.