What I've found is reference to this in the book of Esther in the Bible, specifically Esther 1:19, and a commentary that I found had this to say: "Here is the law of the Medes and the Persians, which cannot be altered, mentioned earlier in the book. In terms of your experience and mine, this is what in Romans is called the law of sin and death. It cannot be altered nor revoked." (emphasis added). Here is the link http://www.raystedman.org/esther/0038.html
So it would seem that someone would use the reference when wishing to say that something cannot be taken back or changed. Do you think that makes sense? :0)