The term itself deflates the power imparted by the concept. It is believed to have been coined by a reviewer named Roz Kaveney, but was not commonly used until its joking inclusion in The Encyclopedia of Science Fiction by Peter Nicholls.
The term is used in science fiction works as to describe a mysterious object of unknown origin and extreme power. It tends to exhibit strange properties or lacks the expected ones. These create an otherworldly feel to the object and add to a sense of wonder in the story elements centered around it. These unusual properties help to instantly convey that the object is not of conventional origin, but instead is something beyond our experiences.
Two notable literary appearances of the Big Dumb Object serve as clear illustrations. In the novel Sphere by Michael Chriton, a massive globe of material is discovered that reflects everything except for people. The only times it reflects a person are when they are alone and that person has been accepted by the object as ‘worthy’ to harness it’s power. Lazy Guns, a device from Iain M. Bank’s novel Against a Dark Background, are supposed to have a lot of mass, but only a little weight. More strangely, turning them upside down causes the weight to increase by as much as three times normal.
So then what does this mean to you as a writer? Not a whole lot honestly. This term is one that serves to make light of something we as writers are trying to prop up. It is good to understand the literary device and how it can be used to very quickly get across concepts of a thing being alien to our understandings without having to become overly expositional. Know what it is if you need to use it and remember not to put too much bluster behind your own device. It is a fine line between mysterious and silly. Think Sphere vs Hitchhiker’s Guide to the Galaxy. Both have a place, but don’t accidentally use the wrong one in your work.