An iconoclast was once someone with a hammer, not an opinion.
The word is built from two Greek parts: eikon, "image," and klastes, "breaker," from the verb "to break." Etymonline traces it through Late Greek eikonoklastes, Medieval Latin iconoclastes and French iconoclaste into English in the 1590s, where it meant a "breaker or destroyer of images."
The first people it described were factions in the Eastern Church in the 8th and 9th centuries. They destroyed icons and other religious objects because they considered them idols. Later the label was applied to Dutch Protestants of the 16th and 17th centuries, who vandalized former Catholic churches for similar reasons.
Image destruction itself is much older than the word. A blog post from the University of Richmond points to the Roman practice of damnatio memoriae, which erased a ruler's image after his fall. It cites the Severan Tondo, a painted portrait from about 200 AD in which Geta's face was erased, most likely after his brother Caracalla had him killed in 211. The blog also describes Byzantine figures in Anatolia, dated between the 5th and 9th centuries, whose faces were deliberately scraped away.
The shift to a figurative meaning came in the 19th century. Etymonline records the sense "one who attacks orthodox beliefs or cherished institutions" in 1842. The blog notes that the secular meaning now stands beside the religious one, and that the American Heritage Dictionary lists it first.
The family of words moved in the same direction at different times:
- iconomachy, an older word for the same quarrel, is recorded in the 1580s
- iconoclastic is from the 1640s
- iconoclasm meant physical destruction in 1797 and the figurative sense in 1858
So the metaphor took a long time to settle. For more than two centuries the English word described actual destruction of actual images, and only afterwards did it become a compliment, or an insult, for anyone who breaks with convention.