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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.