What does 'Dull as ditchwater' mean?

If something is as dull as ditchwater, it is incredibly boring. A ditch is a long narrow hole or trench dug to contain water, which is normally a dark, dirty colour and stagnant (when water turns a funny colour and starts to smell bad). (In American English,'things are 'dull as dishwater'.)

Idioms similar to 'Dull as ditchwater'

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