Not capable of being mitigated, softened, or lessened.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
adj. incapable of being mitigated
Word Usage
"Anna should love, and after our love-journey together, go about our separate lives and dine at the public tables, until the advent of her motherhood, would have seemed a terrible strain upon our unmitigable loyalty."