adj. 25B6; adjective UNAPPEASABLE, unpacifiable, unplacatable, unmollifiable, unforgiving; intransigent, inflexible, unyielding, unbending, uncompromising, unrelenting, ruthless, remorseless, merciless, heartless, pitiless, cruel, hard, harsh, stern, tough.
IMPLACABLE
Meaning of IMPLACABLE in English
Concise Oxford thesaurus English vocabulary. Краткий оксфордский словарь английского языка тезаурус. 2012