Pertaining to, or indicating, fanaticism; extravagant in opinions; ultra; unreasonable; excessively enthusiastic, especially on religious subjects; as, fanatic zeal; fanatic notions.(adjective)
source: webster1913
noun (n)
a person motivated by irrational enthusiasm (as for a cause)(noun.person)
Example: A fanatic is one who can't change his mind and won't change the subject.