Although the term “doughboys” was used for infantry soldiers as far back
as the Mexican-American War (1846–48), it is most commonly used
when referring to soldiers in World War I (1914–18). In the Mexican-American War, the cavalry (horse-riding soldiers)
used the term as an insult when referring to the infantry (foot soldiers),
who were considered inferior. One believable theory is that after long
marches, the infantry was covered in dust and dirt, making them resemble the adobe (dried clay) structures built throughout the Southwest.
The cavalry called them “adobe boys”; in American tongue, that translated to “doughboys.”
Another theory states that the term originates during the American
Civil War (1861–65). Military uniforms worn during that war bore
large, globular buttons that resembled balls of dough.
Nearly five million Americans served their country in World War I.
They referred to themselves as doughboys, and the term was eventually
used even in official military communications.