Quixotic comes straight from Don Quixote, the idealistic, delusional knight-errant at the center of Miguel de Cervantes' 1605 Spanish novel. Don Quixote reads so many chivalric romance novels that he loses his grip on reality, decides he's a knight on a holy quest, and rides off on increasingly ridiculous adventures — most famously charging a row of windmills with his lance because he's absolutely certain they're giants (Part One, Chapter Eight). He loses badly and gets thrown from his horse. He never once doubts the mission.

The character caught on fast: within about fifty years of the novel reaching English readers in translation, just saying the name "Don Quixote" was enough to conjure a whole type of person — someone pouring their whole heart into a cause everyone else had already given up on. But the actual adjective quixotic wasn't coined until 1791, nearly two centuries after Cervantes wrote the book — English simply needed a word for that specific flavor of doomed idealism, and Quixote's own name was sitting right there, ready to become one.