Old English bæð "immersing in water, mud, etc.," also "quantity of water, etc., for bathing," from Proto-Germanic *batham (cf. Old Norse bað, Middle Dutch bat, German bad), from PIE root *bhe- "to warm" (cf. Latin fovere "to foment") + Germanic *-thuz suffix indicating "act, process, condition" (cf. birth, death). Original sense was of heating, not immersing in water. The city in Somerset, England (Old English Baðun) was so called from its hot springs. Bath salts attested from 1875 (Dr. Julius Braun, "On the Curative Effects of Baths and Waters").