1837, from bath + tub.
bath (n.) Old English bæð “immersing in water, mud, etc.,” also “quantity of water, etc., for bathing,” from Proto-Germanic batham (cognates: Old Norse bað, Middle Dutch bat, German bad), from root bhe– “to warm” + Germanic –thuz suffix indicating “act, process, condition” (as in 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.
tub (n.) “open wooden vessel made of staves,” late 14c., from Middle Low German, Middle Dutch, or Middle Flemish tubbe, of uncertain origin. Related to Old High German zubar “vessel with two handles, wine vessel,” German Zuber. Considered to be unrelated to Latin tubus; one theory connects it to the root of two based on the number of handles.
Also 17c. slang for “pulpit;” hence tub-thumper (1660s) “speaker or preacher who thumps the pulpit for emphasis.”
