Am I as dull as...

[ortonym]

Dishwater
or as dull as ditchwater
Probably both 😃
But which one do you know/use?
Any more ambiguous phrases out there?

[Babdoc]

It’s ditch. The water in a ditch is usually sluggish or stagnant, hence dull or cloudy in appearance, compared to a fast flowing clear stream.
I imagine modern city dwellers are less familiar with ditches, and have misheard it as dishwater, which makes sense to them as being dull grey or scummy.