fundamentalist (adj.) Look up fundamentalist at Dictionary.com
1920 in the religious sense (as is fundamentalism), from fundamental + -ist. Coined in American English to name a movement among Protestants c.1920-25 based on scriptural inerrancy, etc., and associated with William Jennings Bryan, among others.

Fundamentalist is said (by George McCready Price) to have been first used in print by Curtis Lee Laws (1868-1946), editor of "The Watchman Examiner," a Baptist newspaper. The movement may have roots in the Presbyterian General Assembly of 1910, which drew up a list of five defining qualities of "true believers" which other evangelicals published in a mass-circulation series of books called "The Fundamentals." A World's Christian Fundamentals Association was founded in 1918. The words reached widespread use in the wake of the contentious Northern Baptist Convention of 1922 in Indianapolis.
Fundamentalism is a protest against that rationalistic interpretation of Christianity which seeks to discredit supernaturalism. This rationalism, when full grown, scorns the miracles of the Old Testament, sets aside the virgin birth of our Lord as a thing unbelievable, laughs at the credulity of those who accept many of the New Testament miracles, reduces the resurrection of our Lord to the fact that death did not end his existence, and sweeps away the promises of his second coming as an idle dream. It matters not by what name these modernists are known. The simple fact is that, in robbing Christianity of its supernatural content, they are undermining the very foundations of our holy religion. They boast that they are strengthening the foundations and making Christianity more rational and more acceptable to thoughtful people. Christianity is rooted and grounded in supernaturalism, and when robbed of supernaturalism it ceases to be a religion and becomes an exalted system of ethics. [Laws, "Herald & Presbyter," July 19, 1922]
The original opposition to fundamentalist (within the denominations) was modernist.
A new word has been coined into our vocabulary -- two new words -- 'Fundamentalist' and 'Fundamentalism.' They are not in the dictionaries as yet -- unless in the very latest editions. But they are on everyone's tongue. [Address Delivered at the Opening of the Seminary, Sept. 20, 1922, by Professor Harry Lathrop Reed, "Auburn Seminary Record"]
Applied to other religions, especially Islam, since 1957.