"breaker or destroyer of images," 1590s, from French iconoclaste and directly from Medieval Latin iconoclastes, from Late Greek eikonoklastes, from eikon (genitive eikonos) "image" + klastes "breaker," from klas- past tense stem of klan "to break" (see clastic).
Originally in reference to those in the Eastern Church in 8c. and 9c. whose mobs of followers destroyed icons and other religious objects on the grounds that they were idols. Applied to 16c.-17c. Protestants in Netherlands who vandalized former Catholic churches on similar grounds. Extended sense of "one who attacks orthodox beliefs or cherished institutions" is first attested 1842.