Originally Posted By: BranShea
Yes! that's the origin.
In the text of the link it's appearing twice. And it does mean that something definitely can't be altered. Happens sometimes. It seems not to be in common use in American English, while here it survived in common speech and in the tolarant negative form.
People often say: Oh well.. it's no law of Medes and Persians.

It would be a fairly common saying amongst older Australians in our English, though not sure whether it has been passed on to younger generations. And yes it does come from the book of Daniel and means something that is rigid and fixed. It's often used to criticise some rule or attitude that is unecessarily officious and inflexible.