Eric Partridge, in his book on catch-phrases, says this is not traceable in English before 1946, but long known in German; probably a translation out of the Occupation following WW2. (See also "break a leg" which came out of Austria after WW1, and originated in the flying corps before crossing over to theatre.) So not due to supposed medieval English peasant family bathing habits (which sound like heroic-age Irish habits).