unalterable (adj.)

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"]

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Definitions of unalterable from WordNet

unalterable (adj.)
not capable of being changed or altered;
unalterable resolve
an unalterable ground rule
Synonyms: inalterable
unalterable (adj.)
of a sentence; that cannot be changed;
an unalterable death sentence
unalterable (adj.)
remaining the same for indefinitely long times;
Synonyms: changeless