Nobody really knows the true origin of "doughboys," the name for U.S. troops who fought in Europe during World War I.
The term may be rooted in the Mexican-American War, when American soldiers became smeared with dust during marches across the parched landscape in Mexico, according to history.com and Wikipedia, which cites several authors.
Coated with dust from clay-rich soil, the soldiers appeared like dough covered in flour, leading to them being called "adobes."
"Adobes" turned into "dobies" and then morphed into "doughboys," the story goes.
Another possibility goes back even further, to Continental Army troops who put clay on the piping of their uniforms to keep it white, history.com says. Rain turned the clay into "doughy blobs," which led to the name.
Back in the Mexican-American War, from 1846 to 1848, the brass buttons on soldiers' uniforms resembled flour dumplings or dough cakes, known as "doughboys," according to references cited on Wikipedia. Soldiers on horseback supposedly used the term to mock foot soldiers. They also may have been called that because of flour or pipe clay used to polish their belts.