very boring, or miserable.

old english phrase dating back to the 1700s, which plays on the double meaning of dull (boring/opaque).

modern english and american english also uses the phrase "dull as dishwater", probably evolved through mispronounciation and familiarity with the latter.
"andy has a new girlfriend but i can't understand why, she is as dull as ditchwater"

"wear the purple one, that black dress is dull as ditchwater"
by miss moo March 25, 2006
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