doughboy (n.)

"U.S. soldier," 1864, American English, said to have been in oral use from 1854, or from the Mexican-American War (1847), it is perhaps from resemblance of big buttons on old uniforms to a sort of cookie or biscuit of that name, a boiled dumpling of raised dough (attested from 1680s), but there are other conjectures.

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