indomitable
adjective /ɪnˈdɒmɪtəbl/
  /ɪnˈdɑːmɪtəbl/
 (formal, approving)- not willing to accept defeat, even in a difficult situation; very brave and determined- an indomitable spirit
- an indomitable campaigner for human rights
 Oxford Collocations DictionaryIndomitable is used with these nouns:- spirit
- will
 Word Originmid 17th cent. (in the sense ‘untameable’): from late Latin indomitabilis, from in- ‘not’ + Latin domitare ‘to tame’.