unmerciful adj. + Add to list US //ənˈmɝsɪfəɫ// UK //ʌnmˈɜːsɪfəl// un·mer·ci·ful SynonymsAntonymsEtymology Synonyms merciless Antonyms mercifulOriginMEunmerciful→ENunmerciful From Middle English unmerciful, unmercifull; equivalent to un- + merciful.