"mud transfused with saline or other ingredients at mineral springs, into which patients suffering from rheumatism, etc., immerse themselves," 1798, from mud (n.) + bath (n.).
Entries linking to mud-bath
mud n.
late 14c., mudde, "moist, soft earth," cognate with and probably from Middle Low German mudde, Middle Dutch modde "thick mud," from Proto-Germanic *mud- from PIE *(s)meu-/*mu- [Buck], found in many words denoting "wet" or "dirty" (source also of Greek mydos "damp, moisture," Old Irish muad "cloud," Polish muł "slime," Sanskrit mutra- "urine," Avestan muthra- "excrement, filth"); related to German Schmutz "dirt," which also is used for "mud" in roads, etc., to avoid dreck, which originally meant "excrement." Welsh mwd is from English. The older word is fen.
Meaning "lowest or worst of anything" is from 1580s. As a word for "coffee," it is hobo slang from 1925; as a word for "opium" from 1922. Mud-puppy "salamander" is by 1855, American English; the mud-dauber wasp was so called by 1856. The children's mud-pie is attested from 1788. Mud-flat "muddy, low-lying ground near a shore" is by 1779. Mud-room "room for removing wet or muddy footwear" is by 1938.
The expression clear as mud (that is, "not clear at all") is by 1796. To throw or hurl mud "make disgraceful accusations" is from 1762. To say (one's) name is mud and mean "(one) is discredited" is recorded from 1823, from mud in obsolete sense of "a stupid twaddling fellow" (1708). Mud in your eye as a toast is recorded from 1912, American English.
bath n.
Old English bæð "an immersing of the body in water, mud, etc.," also "a quantity of water, etc., for bathing," from Proto-Germanic *badan (source also of Old Frisian beth, Old Saxon bath, Old Norse bað, Middle Dutch bat, German Bad), from PIE root *bhē- "to warm" + *-thuz, Germanic suffix indicating "act, process, condition" (as in birth, death). The etymological sense is of heating, not immersing.
The city in Somerset, England (Old English Baðun) was so called from its hot springs. Bath salts is attested from 1875 (Dr. Julius Braun, "On the Curative Effects of Baths and Waters"). Bath-house is from 1705; bath-towel is from 1958.