The Encyclopedia Americana (1998, vol.9, p.306) says that "doughboy" is a slang term for a soldier of the U.S. Army in WWI. It was first recorded as it applied to soldiers in 1867, a derogatory term applied to infantrymen, particularly by cavalrymen. The most commonly accepted origin of the word lies in the fact that the Union infantry uniform in the Civil War had large globular brass buttons resembling a doughboy, which was a flour dumpling cooked in soup.