"It was as dull as a dishwasher."
My friends’ giggly response suggested that what I had said wasn’t quite right. Whilst this isn’t as unusual as I’d perhaps like to admit on this blog, I hadn’t expected the ensuing squabble over what the correct phrase actually is. One option was "as dull as dishwater", while the other was "as dull as ditchwater". I then realised that my version of the expression, although it made sense in a way, lacked the play on the double meaning of dull (boring and opaque) found in the more received expressions above. No-one was sure, so I thought I’d look it up.
I first turned to my trusted Robert & Collins, which told me: "as dull as ditchwater or dishwater". The Cambridge International Dictionary of Idioms agreed that both were used. The OED has en entry for "as dull as ditchwater", but not for the other one. Finally, the American Heritage® Dictionary of Idioms tells us that although both forms are now in use, the original is "dull as ditchwater", dating from the 1700s, which only became dishwater in the first half of the 1900s, probably through mispronunciation.
I’ll hazard a guess here: maybe, as populations became more urbanised, ditchwater turned into dishwater because washing the dishes was a much more familiar experience than peering into ditches, which are rather rare in towns. I bet you that before long, as dishwashers become more and more common in households, dishwater will turn into dishwasher, and then no-one will laugh at me because of it. Although they will probably have found something else.
In French? I’d say ennuyeux comme la pluie (as dull as the rain).