immitigable
Concise Oxford English Dictionary © 2008 Oxford University Press:
immitigable /ɪˈmɪtɪgəb(ə)l/
▶adjective chiefly literary unable to be made less serious.
– derivatives
immitigably adverb.
immitigably adverb.
– origin C16: from late L. immitigabilis, from in- ‘not’ + mitigabilis ‘able to be mitigated’.
'immitigable' also found in these Oxford entries:

