Borrowed from French doctrinaire, from doctrine + -aire.

doctrinaire (plural doctrinaires)

1. A person who stubbornly holds to a philosophy or opinion regardless of its feasibility.
   • 1905 April, Jack London, “A Review”, in War of the Classes, New York, N.Y.: The Macmillan Company; London: Macmillan & Co., Ltd., →OCLC ↗, page 212 ↗:

     And he is not above feeling grave and well-contained satisfaction wherever the socialist doctrinaire has been contradicted by men attempting to practise coöperation in the midst of the competitive system, as in Belgium.

   • 1909 April, O. Henry [pseudonym; William Sydney Porter], “The Renaissance at Charleroi”, in Roads of Destiny, Garden City, N.Y.: Doubleday, Page & Company, →OCLC ↗, page 274 ↗:

     Or, shall we play the doctrinaire, and hint that at thirty-four the tides of life are calmer and cognizant of many sources instead of but one—as at four-and-twenty?

2. (historical) In France, in 1815–30, one of a school who desired a constitution like that of Britain.

• German: Prinzipienreiter, Doktrinär

doctrinaire

1. Stubbornly holding on to an idea without concern for practicalities or reality.
   • [1891, Oscar Wilde, The Picture of Dorian Gray, London, New York, N.Y., Melbourne, Vic.: Ward Lock & Co., →OCLC ↗:

     Mr. Chapman began to talk in a loud voice about the situation in the House of Commons. He guffawed at his adversaries. The word doctrinaire—word full of terror to the British mind—reappeared from time to time between his explosions.]

   • 1911, Bram Stoker, The Lair of the White Worm, London: William Rider and Son, […], →OCLC ↗:

     We have been long accustomed to consider growth as applied almost exclusively to size in its various aspects. But Nature, who has no doctrinaire ideas, may equally apply it to concentration.

• German: prinzipienreiterisch, doktrinär