If someone or something is as dull as ditchwater or as dull as dishwater, they are very boring. He's a dull writer and that's a fact. Dull as ditchwater. `Most of the politicians in Ontario are dull as dishwater,' he said. Note: The expression `dull as ditchwater' is over 200 years old, whereas `dull as dishwater' is a more recent variant. The reference is to the dull dirty colour of the water in ditches or in washing-up bowls.