One of the prominent female characters in Tasso's Jerusalem Delivered. She was a beautiful sorceress, with whom Rinaldo fell in love, and wasted his time in voluptuous pleasure. Two messengers were sent from the Christian army with a talisman to disenchant him. After his escape, Armida followed him in distraction, but not being able to allure him back, set fire to her palace, rushed into the midst of a combat, and was slain.
In 1806, Frederick William of Prussia declared war against Napoleon, and his young queen rode about in military costume to arouse the enthusiasm of the people. When Napoleon was told of it, he wittily said of her: “She is Armida, in her distraction setting fire to her own palace.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Armi da from Infoplease: