When it is capitalized, this word refers to the mineral springs in Liège, Belgium, believed to have curative properties. In the last several centuries the meaning of the word has been generalized to refer to almost any place that has curative mineral waters, according to OED, so that “a town, locality, or resort possessing a mineral spring or springs; a watering-place of this kind” is fairly known as a spa.
The generalization has expanded further in recent decades through commercialization, and as always OED’s tracking of the etymology is intriguing:
A commercial establishment which offers health and beauty treatment (esp. for women) through steam baths, exercise equipment, massage, and the like. U.S.
1960 Life 8 Feb. 111/1 The submerged specter above‥is getting a hydraulic underwater massage at a plush health spa near San Diego called the Golden Gate beauty resort whose customers are usually female.1976 Vogue Dec. 214/1 Most American spas are designed exclusively for women.1981 W. Safire in N.Y. Times Mag. 21 June 10/2 Only fuddy-duddies go to the gym,‥the upscale‥crowd goes to the spa.


