Wordorigins.org

This name for an American soldier is of unknown origin. Despite being popularly associated with the First World War, doughboy actually dates to the Mexican-American War in 1847. From N.J.T. Dana’s Monterrey of that year:

We “doughboys” had to wait for the artillery to get their carriages over.

Several conjectures about the origin have been advanced, but none have any real evidence to support them.

One such conjecture is that the buttons on Civil War uniforms resembled a type of dumpling called a doughboy. While the dumpling sense of doughboy dates to the 17th century, the use of the word to mean a soldier predates the Civil War by several decades. From Ringrose’s Bucaniers of America from 1685:

These men...had each of them three or four Cakes of bread (called by the English Dough-boy’s) for their provision and Victuals.

A second is that infantryman in the 1850s wore white belts and used flour to cover nicks and scratches. Again, the date is off, but is getting closer.

And a third is that it is from the dust-caked uniforms from marching through Mexico, the dough- being a variant of adobe.

(Sources: Historical Dictionary of American Slang; Oxford English Dictionary, 2nd Edition)