adj. Incapable of being modified; immune to modification.
The Century Dictionary and Cyclopedia
Not modifiable; not capable of being modified.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
adj. incapable of being modified in form or character or strength (especially by making less extreme)
Word Usage
"It's also worth remembering that this "unmodifiable" pro-capitalist human nature, far from being inherent in the human character, defies the historical record."