That cannot be mitigated: an immitigable loss.

im·mit′i·ga·bil′i·ty n.
im·mit′i·ga·bly adv.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
References in classic literature ?
From that hour of evil omen until the present, it may be,--though we know not the secret of his heart,--but it may be that no wearier and sadder man had ever sunk into the chair than this same Judge Pyncheon, whom we have just beheld so immitigably hard and resolute.
(12) No original: "The democratic consistency, consummately and immitigably complete, shines through with its hard light, whatever equivocal gloss may happen momentarily to prevail".
(33) Like Benjamin, Olson notices that the only thing that remains after the demolition of experience caused by modern warfare is "the tiny, fragile human body." If there is any chance for experience to emerge again as a cultural force, he asserts, the immitigably vulnerable atom of the human body will be its basis--an atom that must build up social molecules with painstaking care because it has become unmoored from the hapless detritus of culture that surrounds it.