The story
Sense 2: from earth + bathing. Widely understood to have been coined Scottish physician James Graham in the late 1780s or early 1790s. * Possibly coined earlier by Gerard van Swieten in 1765 in references to practices found in the Kingdom of Granada (writing "per balneum terrae," Latin for "bath of earth").
Etymology from Wiktionary, CC BY-SA 4.0.