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Webster 1828 Edition

adv.

Unchangeably; unalterably; invariably; in a manner that admits of no change.

immutably ‎(comparative more immutably, superlative most immutably)

In an immutable manner. In a way that cannot be varied, or changed.
- (Can we date this quote?), Charlotte Brontë, chapter 33, in Jane Eyre:
  My task was a very hard one; but, as I was absolutely resolved - as my cousins saw at length that my mind was really and immutably fixed on making a just division of the property....they yielded at length.