unalterable (adj.)youdaoicibaDictYouDict[unalterable 词源字典]
1610s, from un- (1) "not" + alterable. Related: Unalterably.
He reach'd a middle height, and at the stars,
Which are the brain of heaven, he look'd, and sank.
Around the ancient track march'd, rank on rank,
The army of unalterable law.

[George Meredith, "Lucifer in Starlight"]
[unalterable etymology, unalterable origin, 英语词源]