A doughboy is old American slang for a US soldier. The term dates back to the Mexican-American War and had been used with frequency through WWI, after which ‘GI’ was more commonly used. Where does the term doughboy come from?
No one knows for sure. There are a few common folk etymologies, though. One states that while the American troops were hiking through the deserts of Northern Mexico they would become caked with dust the color of dough. Another hypothesis references the big buttons on their uniforms. Apparently they looked like big biscuits that were called doughboys back then.
The unfortunate truth is that no one knows the origin of the term doughboy.