1
general::
Phrase(s): *dull as dishwater [and] *dull as ditchwater
very uninteresting. (*Also: as ~.) • I’m not surprised that he can’t find a partner. He’s as dull as dishwater. • Mr. Black’s speech was as dull as dishwater.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Boring, tedious, as in That lecture was dull as dishwater. The original simile, dull as ditchwater, dating from the 1700s, alluded to the muddy water in roadside ditches. In the first half of the 1900s, perhaps through mispronunciation, it became dishwater, that is, the dingy, grayish water in which dirty dishes had soaked.
American Heritage Idioms