Wiktionary
unmitigable
a. Not able to be mitigated; not mitigable.
WordNet
unmitigable
adj. incapable of being mitigated; "stern and unmitigable accusations"
Usage examples of "unmitigable".
If my lady is not included in the Set, she will want to leave Court and then I shall be sent back to be maidservant to the unmitigable Dowager Marchioness of Netherby-on-the-Fens!
O, worn and beating heart, may I dissect thy fibres, and tell how in each unmitigable misery, sadness dire, repinings, and despair, existed?