A person who contributes little or nothing to a given situation. Although you find galleywackers everywhere those in the workplace are the most annoying because they are likely receiving the same or greater rewards while doing little of the actual work.
Jeff was exactly right when he said the Roberts crew was a bunch of galleywackers.
A rather simple course one can study at Uppsala University in Sweden. You might however find it to be extremely challenging if your name is David Dahlgren.