Is the normal expression 'as dim as dishwater' or 'as dim as ditchwater?
When you google this it comes up as 'dull as dishwater/ditchwater'.
There is a difference between being 'dim' and being 'dull'.
The former means lacking in intelligence and sharpness, what Americans might call 'dumb'. The latter means uninteresting.
So which is it - dim as dishwater/ditchwater - dull as dishwater/ditchwater?