That cannot be mitigated; implacable, uncompromising.
Origin
Early 17th century; earliest use found in William Shakespeare (1564–1616), playwright and poet. Alteration of immitigable, with substitution of un- for the first element.
Definition of unmitigable in US English:
unmitigable
adjective(ˌ)ʌnˈmɪtɪɡəbl
That cannot be mitigated; implacable, uncompromising.
Origin
Early 17th century; earliest use found in William Shakespeare (1564–1616), playwright and poet. Alteration of immitigable, with substitution of un- for the first element.