Possessed with or motivated by excessive, irrational zeal.(adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "fanatical" in a sentence
"Its no coincidence that the word fanatical comes up so often when talking about this companys customer service ethic."
"You know they are extremely verdant, as well as what you term fanatical, and they are not likely to make any capital out of such"
"You call it fanatical, and so does the world; but it has been, and ever is, the most comfortable animating doctrine of the Jewish church, from David "the sweet Psalmist of Israel," and of the Church Of CHRIST, from its institution."