“The law of the Medes and Persians”
establish the decree and sign the document so that it cannot be changed, in accordance with the law of the Medes and Persians, which cannot be repealed. (Daniel 6:8 BSB)
What it sounds like today
A generic bit of legal boilerplate.
The cultural key
The whole lions' den crisis turns on this legal principle. In the Medo-Persian empire a decree signed and sealed by the king became irrevocable, part of 'the law of the Medes and Persians, which cannot be repealed', binding even on the king who made it. Daniel's jealous rivals weaponize this: they flatter Darius into signing an irreversible thirty-day law that anyone who prays to any god or man except the king will be thrown to the lions, knowing full well that Daniel prays to the LORD three times a day. Once Darius realizes he has been tricked into condemning his most trusted official, he desperately tries to rescue Daniel but legally cannot, the law he signed cannot be unsigned, so Daniel goes to the lions. The identical fixed feature of Persian law drives the book of Esther, where an anti-Jewish decree cannot be revoked and must be countered instead. A modern reader skims 'cannot be repealed' as boilerplate; it is the trap the entire chapter springs.
Recovered meaning
A signed Medo-Persian decree could not be revoked, even by the king, and Daniel's enemies exploit that to force Darius to send him to the lions against his will. The irreversible-law custom is the engine of the plot, as it is in Esther.