This is an English idiom which is derived from an episode in the novel Don Quixote written by the Spanish novelist Miguel de Cervantes.
In the novel, the protagonist (the main character in a play, story, film, etc.), Don Quixote, an aging gentleman who is obsessed with stories of knights and chivalry, loses his sense of reality and decides to become a wandering knight himself.
He mounts his broken down old horse, picks a local stupid and unpleasant fellow, Sancho Panza, as his assistant or squire and sets out to battle dragons and rescue maidens and other such acts of bravery.
At one point he rides up to windmills that he sees as giants and decides to fight them.
He tells Sancho Panza:
“Fortune is guiding out affairs better than we ourselves could have wished. Do you see over yonder, friend Sancho, thirty or forty hulking giants? I intend to do battle with them and slay them. With their spoils we shall begin to be rich for this is a righteous war and the removal of so foul a brood from off the face of the earth is a service God will bless.”
‘What giants?” asks Sancho Panza.
“Those you see over there,” replies his master, “with their long arms. Some of them have arms well nigh two leagues in length.”
In spite of the fact that Sancho Panza warns him that those structures are not giants but windmills, Don Quixote attacks them with his lancer, with very devastating consequences.
It was towards the end of the 19th century that the expression came into use.
For example, in The New York Times, April 1870.
“They have not thus far had sufficient of an organization behind them to make their opposition to the committee’s bill anything more than tilting at windmills.”
The expression; Tilting at windmills means engaging in a futile activity or attacking imaginary enemies.
Example: Your attempt at reforming the system is more like tilting at windmills.
