(Will appear in the August 2008 issue of Community News)
Nowadays, this generally refers to a person who doesn't do his part, a loafer, someone who shirks his work. In earlier days, especially during the World Wars, it usually referred to a soldier who didn't do his part of the work. It can also refer to an investment that looks good but turns out to be worthless. Supposedly, the word originated in late 19th-century America, when people were fooled into buying bricks of gold that were only gold on the outside. In World War One, new recruits were sometimes promoted to lieutenant before they knew what they were doing, earning the scorn of their men and being called goldbricks because of the color and shape of their insignia. From there, the term became general, first for lazy soldiers and then for lazy civilians. Personally, I think this story smacks of folk etymology and we'd know the real origin of this word if the etymologists would just stop goldbricking.
I'm collecting all of these at: http://www.dvorkin.com/davidsdefs.html