"(as|) dull as dishwater" 的定义和意义

What is the origin of the idiom "dull as dishwater" and when to use it?

The idiom "dull as dishwater" originated in the 1800s as "dull as ditchwater," which alluded to the muddy water found in ditches. The phrase "dull as dishwater" seems to have overtaken the original expression in popularity, possibly due to a pronunciation or printing error. The color of dishwater, being a dull gray, is related to the other meaning of "dull," signifying something mundane. The original phrase, "dull as ditchwater," and the newer phrase have similar meanings, both conveying a sense of extreme boredom and lack of interest. The idiom is now used to describe something that is very boring and tedious, and it has been in use since the 1700s.