为什么步兵是“doughboy”呢？是不是在美国的某个时期步兵的津贴要高些呢？

Word that was universally used in the U.S. Army to mean an infantryman, and specifically an American infantryman, up until World War II, when it was replaced with "GI." When it was first used is uncertain, but it can be traced as far back as 1854, when it was already in use on the Texas border, and it was especially popular in World War I. The explanation then was that the infantrymen wore white belts and had to clean them with "dough" made of pipe clay. Originally a term of ridicule used by the mounted service, it was adopted by the infantry itself and used with great pride.

步兵是infantry

楼上说得对！

可能doughboy只是某些变态中学阅读理解里面出现的东西吧！