the GNU version of the Collaborative International Dictionary of English
adj. Not capable of being mitigated, softened, or appeased.
The Century Dictionary and Cyclopedia
Not mitigable; incapable of being mitigated or appeased.
Word Usage
"Her main fault was a brooding, eternal, immitigable suspicion of all men, things, creeds, and parties; this suspicion was a mist before her eyes, a false guide in her path, wherever she looked, wherever she turned."