(UK) 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'.)
If something is sold by setting a price, then reducing it until someone buys it, it is sold in a Dutch auction. It can also mean that something is changed until it is accepted by everyone.
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