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"