(as) dull as dishwater

Very boring or unexciting. This phrase is often used to describe a person. My date with Dave was not great—he is as dull as dishwater. I fell asleep during that movie because it was as dull as dishwater.

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.

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.

Flat, boring. This expression began life in the eighteenth century as dull as ditchwater, alluding to the muddy color of the water in roadside gullies. “He’d be sharper than a serpent’s tooth, if he wasn’t as dull as ditchwater,” says Dickens’s Fanny Cleaver (Oliver Twist). This version survived on both sides of the Atlantic well into the twentieth century. Either through careless pronunciation or through similar analogy it occasionally became dishwater—water in which dishes had been washed and which consequently was dingy and grayish.