Why is the ninth month called September?
1540s, verbal noun from bathe (v.). Bathing suit is recorded from 1852 (bathing costume from 1830); bathing beauty is 1920, from vaudeville.
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").
n. pl. baths (bāðz, bāths)
The act of soaking or cleansing the body or any of its parts, as in water.
The apparatus used in giving a bath.
The fluid used to maintain the metabolic activities of an organism.
a Hebrew liquid measure, the tenth part of an homer (1 Kings 7:26, 38; Ezek. 45:10, 14). It contained 8 gallons 3 quarts of our measure. "Ten acres of vineyard shall yield one bath" (Isa. 5:10) denotes great unproductiveness.