The phrase "tilt at windmills" comes from a scene in "The Ingenious Nobleman of La Mancha," also titled "Don Quixote," a novel first published in 1605 by Spanish author Miguel de Cervantes. The main character, Don Quixote, acting like a knight long after the days of chivalry are over, "tilts at," or attacks, a group of windmills he calls giants. But the wind spins the windmills' vanes, breaking Don Quixote's lance and throwing him and his horse to the ground.
The phrase means to fight an imaginary enemy or to fight an enemy that cannot be beaten. Newspaper columnist Evan Morris notes that it also implies a victory is unlikely or unrealistic even though the battle itself may be noble or have a noble goal. In the case of Don Quixote, his delusions of living in a mythic age are appealing enough to draw in his friends despite the hopelessness of his quest.
This classic novel about idealism versus realism also spawned the word "quixotic," an adjective used to describe someone who is foolishly idealistic in an impractical yet chivalrous way. The story has been made into a ballet, films and a musical, making Don Quixote and his windmills a lasting symbol of lost causes.